VintageBigBlue.org

 

Re: [funwithtransistors] Re: More on the class AB transistor amp:


Jan 17, 2011

 


----------------------------

#337 Jan 17, 2011

i1094.photobucket.com/albums/i443/Der_Hellste_Stern/TransistorAmp.gif?t=1295293103



Hello all.



I set up this amplifier this morning on a breadboard. I was less than satisfied with its performance. The sound's volume was okay but required the volume to be set to maximum; anything lower than maximum made it awfully quiet.



I drove this with an iPod, putting out a 1KHz sine wave. It was a 5 second sample played on infinite loop. The maximum signal envelope was about 1.5 vpp, but the iPod surprisingly clipped above ~1.1vpp.



The first preamp transistor would take the input up to 6vpp before clipping. Strangely, the sine waves did not square off but rounded off instead. This took place just above 6.1 vpp.



The driver transistor clipped just above 2.5vpp. Tweaking with the 2.7K resistor in its collector path and the 1K resistor in its emitter allowed me to get the unclipped output up to 3.8vpp. Adjusting the rubber diode transistor did not do much for me, although crossover distortion showed up when I turned the transistor completely off. I biased the output transistors at 20 mA.



For 1 watt of output, I need the signal to vary 8 vpp across an 8-ohm speaker. The whole amplifier at the moment is incapable of this, but I am more concerned with the driver transistor than the preamplifier. If I can get the driver to play ball, then I at least have 6 vpp to work with.



Thanks for your help. I have been toying with this for some time and now I'm making some head way.



Ed



----------------------------

#338 Jan 17, 2011

--- On Mon, 1/17/11, Der_Hellste_Stern lordof16bit@...> wrote:

> From: Der_Hellste_Stern lordof16bit@...>

> Subject: [funwithtransistors] More on the class AB transistor amp:

> To: funwithtransistors@yahoogroups.com

> Date: Monday, January 17, 2011, 3:01 PM

> i1094.photobucket.com/albums/i443/Der_Hellste_Stern/TransistorAmp.gif?t=1295293103

>

> Hello all.

>

> I set up this amplifier this morning on a breadboard. I was

> less than satisfied with its performance. The sound's volume

> was okay but required the volume to be set to maximum;

> anything lower than maximum made it awfully quiet.



Is there a reason that the emitter resistors aren't bypassed? Doesn't this

inject some negative gain into the circuit?

> I drove this with an iPod, putting out a 1KHz sine wave. It

> was a 5 second sample played on infinite loop. The maximum

> signal envelope was about 1.5 vpp, but the iPod surprisingly

> clipped above ~1.1vpp.



That doesn't surprise me. :*) 1 Volt is about all you can expect out of such

devices, as a general rule (The other general rule is to never believe a general

rule!).

> The first preamp transistor would take the input up to 6vpp

> before clipping. Strangely, the sine waves did not square

> off but rounded off instead. This took place just above 6.1

> vpp.



Rounded off?

> The driver transistor clipped just above 2.5vpp. Tweaking

> with the 2.7K resistor in its collector path and the 1K

> resistor in its emitter allowed me to get the unclipped

> output up to 3.8vpp. Adjusting the rubber diode transistor

> did not do much for me, although crossover distortion showed

> up when I turned the transistor completely off. I biased the

> output transistors at 20 mA.



That sounds reasonable.

> For 1 watt of output, I need the signal to vary 8 vpp

> across an 8-ohm speaker. The whole amplifier at the moment

> is incapable of this, but I am more concerned with the

> driver transistor than the preamplifier. If I can get the

> driver to play ball, then I at least have 6 vpp to work

> with.



Have you run a SPICE simulation?

> Thanks for your help. I have been toying with this for some

> time and now I'm making some head way.



Headway is good.

> Ed



Dave



----------------------------

#339 Jan 17, 2011

Hi Dave,

> Is there a reason that the emitter resistors aren't bypassed? Doesn't this inject some negative gain into the circuit?



That might be what I am experiencing, why I'm not getting the gain I should be. Emitter bypass lowers the input impedance and increases gain. The former effect doesn't happen with tubes, but I want to save as much Zin as I can. I might port this design to a guitar amplifier some time. As for boosting gain, I don't know how to control the increase of gain and I don't want the signal to be distorted. I want to see as little clipping as I can. If I had a distortion meter maybe I'd be a little happier, or at least a little more obsessive about this.

> That doesn't surprise me. :*) 1 Volt is about all you can expect out of such devices, as a general rule (The other general rule is to never believe a general rule!).



The output level of the iPod was fine. I used to simulate line-level output at 0.1 volts peak. Good thing I found out what it really is.

> Rounded off?



I say this because the tops of the waveforms weren't perfect, clean squares. They also weren't perfect sine peaks either. It was like they were dulled and rounded off. Next time I do this I might end up taking o-scope photos. I can't really explain it clearer than that.

> That sounds reasonable.



Good. I thought I was crazy. Except I am.

> Have you run a SPICE simulation?



No. I'm not good with SPICE and I find it more instructive for me to sit down with the actual parts and run real experiment. This way I can also get a feel for unintended things, like possible noise/hum pickup and how hot the power transistors will get. SPICE assumes too much for me to spend the time playing with it.

> Headway is good.



Yes, but I won't be satisfied until I get 1 watt. Then it will be 5 watts. You can see a pattern developing. Am I crazy to want to achieve 1 watt with a 12 VDC source? I am thinking to stop wasting my time with a mere 1 watt amplifier and go ahead and look for 8-10 watts.



Ed







----------------------------

#340 Jan 18, 2011

Ed.



It would help to split the 2.7 k resistor into two resistors and boot strap

it from the output or instead of a resistor use a PNP as a current source.



Regards.



Max. K 4 O D S.



Email: max@...



Transistor site www.funwithtransistors.net

Vacuum tube site: www.funwithtubes.net

Music site: www.maxsmusicplace.com



To subscribe to the fun with transistors group send an email to.

funwithtransistors-subscribe@yahoogroups.com



To subscribe to the fun with tubes group send an email to,

funwithtubes-subscribe@yahoogroups.com

----- Original Message -----

From: "Der_Hellste_Stern" lordof16bit@...>

To: funwithtransistors@yahoogroups.com>

Sent: Monday, January 17, 2011 2:01 PM

Subject: [funwithtransistors] More on the class AB transistor amp:





> i1094.photobucket.com/albums/i443/Der_Hellste_Stern/TransistorAmp.gif?t=1295293103

>

> Hello all.

>

> I set up this amplifier this morning on a breadboard. I was less than

> satisfied with its performance. The sound's volume was okay but required

> the volume to be set to maximum; anything lower than maximum made it

> awfully quiet.

>

> I drove this with an iPod, putting out a 1KHz sine wave. It was a 5 second

> sample played on infinite loop. The maximum signal envelope was about 1.5

> vpp, but the iPod surprisingly clipped above ~1.1vpp.

>

> The first preamp transistor would take the input up to 6vpp before

> clipping. Strangely, the sine waves did not square off but rounded off

> instead. This took place just above 6.1 vpp.

>

> The driver transistor clipped just above 2.5vpp. Tweaking with the 2.7K

> resistor in its collector path and the 1K resistor in its emitter allowed

> me to get the unclipped output up to 3.8vpp. Adjusting the rubber diode

> transistor did not do much for me, although crossover distortion showed up

> when I turned the transistor completely off. I biased the output

> transistors at 20 mA.

>

> For 1 watt of output, I need the signal to vary 8 vpp across an 8-ohm

> speaker. The whole amplifier at the moment is incapable of this, but I am

> more concerned with the driver transistor than the preamplifier. If I can

> get the driver to play ball, then I at least have 6 vpp to work with.

>

> Thanks for your help. I have been toying with this for some time and now

> I'm making some head way.

>

> Ed

>

>

>

---------------

>

> Always keep electrons and holes flowing in opposite directions.Yahoo!

> Groups Links

>

>

>

>



----------------------------

#341 Jan 18, 2011

This helps a lot. I also wondered if the following things would work:



;Instead of powering the driver transistor's base network from the 12V rail (I refer to the 56K resistor in series with the 15K), connecting the top of the 56K to the collector of that transistor. Alternately, I could attach it to the output where the emitters of the power transistors meet.



;I might increase my max. signal swing by bypassing the 1K emitter resistor on the driver. Right now, that leg adds a parallel path for input impedance of B * Re, or 200K. I don't know how low the path would get if I bypassed it.



I wonder some things about your suggestions though:



;If I boot-strapped the 2K7 resistor, what kind of ratio do I need to use for the division? The easiest combination I can think of off-hand is a 2K2 resistor and a 470-ohm one. Also, I'm not one-hundred percent sure I know how to calculate the increase of gain. Normally in situations like that I just hook it up and see what happens. The driver clips now, so making a change that caused it to clip more would not irritate me.



;What kind of current figure am I looking at for my current source? I think the resistor-driver set drew about 3 mA. I think the output transistor current has less to do with the current in the driver circuit and more to do with the voltage difference between the two bases. I may end up taking out the "rubber diode" and trim pot, replacing it with three silicon diodes. I eventually want to eliminate the 1r resistor in the path of the output transistors, but that will come after I'm sure they won't run away on me.



Another oddity I noticed: I was watching the collector of the driver transistor while playing with the iPod volume control. The test waveform did something strange. I increased the signal to a point just below clipping. A small bump appeared on the tops of the positive excursions of the collector wave. It was reminiscent of cartoons where a character would hit their head and the bump would grow out of their hair. I observed the same effect on a triangle wave, but a small square pulse grew out of the top just before clipping. Square waves didn't do this, but that's a different story altogether.



Thanks

Ed



----------------------------

#342 Jan 18, 2011

Der_Hellste_Stern wrote:



I'm more accustomed to seeing that 1 ohm resistor split between

the two emitters.



[...]

> The first preamp transistor would take the input up to 6vpp before

> clipping. Strangely, the sine waves did not square off but rounded

> off instead. This took place just above 6.1 vpp.



You've got a collector current of about 7 mA and a collector

voltage of about 5.1 V. You've got a Vce of about 4.4 V.

If you cut the transistor completely off, then the upper

peak would be 12.2 V. If you saturated the transistor, the

collector voltage would be about 1.1 V. So, your P-P it

can produce driven to the max would be about 11 V P-P.



Sounds like "soft clipping". That usually happens when one is

reaching current levels where the Hfe starts to fall off on

the transistors, or the power supply is starting to "sag".

You may either need a "stiffer" power supply, or you may need

more driver gain, like with Darlington outputs.



However, you say that you are clipping at the driver? Not just

the output? That points to power supply. Were these tests conducted

with a load on the output, or unloaded?



Have you 'scoped your power supply lines while you are clipping?



Mac

--

p="p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}

Oppose globalization and One World Governments like the UN.

This message made from 100% recycled bits.

You have found the bank of Larn.

I speak only for myself, and I am unanimous in that!







----------------------------

#343 Jan 18, 2011

Hi Ed.



Biasing the base of the driver through resistors from its collector adds

local negative feedback around that stage. Its major effect will be to

stabilize the DC operating point. The stability factor is given by S =

Rb/(Rc + Re). Where Rb is the Thevenin equivalent of the voltage divider in

the base. A stability factor of 10 is generally acceptable for a home

environment where the amplifier won't need to operate over a wide

temperature range. For portable and mobile equipment S should be around 2

or 3. The collector of the driver and the output are essentially the same

point for DC and AC. The AC NFB will lower the gain of the driver and help

to linearize it.



Bypassing the emitter would give a little extra negative swing because the

emitter would not be rising up to meet the collector part way.



Bootstrapping is positive feedback so you do have to be careful with it.

Too much and you've got an oscillator. The values you suggest should be

well within safe limits. The 470 ohm resistor should be the one with one

end connected to the positive rail.



Gain calculations can be difficult because they require knowing the AC

parameters of the transistor.



The current source should be the same as the 2.7 k resistor draws now. If

you are using a 12 volt power supply and the amplifier is biased at 6 volts,

as it should be for maximum swing, the current would be I = 6/2700 = 2.2 mA.

With a current source you will need DC feedback around the driver at least

to set and stabilize the operating point.



Then you wrote.

>I think the output transistor current has less to do with the current in

>the driver circuit and more to do >with the voltage difference between the

>two bases.



That's right.

>I may end up taking out the "rubber diode" and trim pot, replacing it with

>three silicon diodes. I >eventually want to eliminate the 1r resistor in

>the path of the output transistors, but that will come after >I'm sure they

>won't run away on me.



Eliminating the 1 ohm resistor is unwise. Transistors can always run away

especially in this configuration.



The artifact you are seeing is probably caused by the rubber diode unloading

the collector just before the voltage gets to the positive rail.



Regards.



Max. K 4 O D S.



Email: max@...



Transistor site www.funwithtransistors.net

Vacuum tube site: www.funwithtubes.net

Music site: www.maxsmusicplace.com



To subscribe to the fun with transistors group send an email to.

funwithtransistors-subscribe@yahoogroups.com



To subscribe to the fun with tubes group send an email to,

funwithtubes-subscribe@yahoogroups.com

----- Original Message -----

From: "Der_Hellste_Stern" lordof16bit@...>

To: funwithtransistors@yahoogroups.com>

Sent: Tuesday, January 18, 2011 6:09 AM

Subject: [funwithtransistors] Re: More on the class AB transistor amp:





> This helps a lot. I also wondered if the following things would work:

>

> ;Instead of powering the driver transistor's base network from the 12V

> rail (I refer to the 56K resistor in series with the 15K), connecting the

> top of the 56K to the collector of that transistor. Alternately, I could

> attach it to the output where the emitters of the power transistors meet.

>

> ;I might increase my max. signal swing by bypassing the 1K emitter

> resistor on the driver. Right now, that leg adds a parallel path for input

> impedance of B * Re, or 200K. I don't know how low the path would get if I

> bypassed it.

>

> I wonder some things about your suggestions though:

>

> ;If I boot-strapped the 2K7 resistor, what kind of ratio do I need to use

> for the division? The easiest combination I can think of off-hand is a 2K2

> resistor and a 470-ohm one. Also, I'm not one-hundred percent sure I know

> how to calculate the increase of gain. Normally in situations like that I

> just hook it up and see what happens. The driver clips now, so making a

> change that caused it to clip more would not irritate me.

>

> ;What kind of current figure am I looking at for my current source? I

> think the resistor-driver set drew about 3 mA. I think the output

> transistor current has less to do with the current in the driver circuit

> and more to do with the voltage difference between the two bases. I may

> end up taking out the "rubber diode" and trim pot, replacing it with three

> silicon diodes. I eventually want to eliminate the 1r resistor in the path

> of the output transistors, but that will come after I'm sure they won't

> run away on me.

>

> Another oddity I noticed: I was watching the collector of the driver

> transistor while playing with the iPod volume control. The test waveform

> did something strange. I increased the signal to a point just below

> clipping. A small bump appeared on the tops of the positive excursions of

> the collector wave. It was reminiscent of cartoons where a character would

> hit their head and the bump would grow out of their hair. I observed the

> same effect on a triangle wave, but a small square pulse grew out of the

> top just before clipping. Square waves didn't do this, but that's a

> different story altogether.

>

> Thanks

> Ed

>

>

>

>

---------------

>

> Always keep electrons and holes flowing in opposite directions.Yahoo!

> Groups Links

>

>

>

>







----------------------------

#344 Jan 18, 2011

Hello again,



Max writes:

>Biasing the base of the driver through resistors from its collector adds local negative feedback around that stage. Its major effect will be to stabilize the DC operating point.



Which is good. I hate having to recalculate the entire driver when I want to change the value of one resistor. I am hoping that this configuration acts like some sort of "autobias". By this term, I mean a self-adjusting DC network sort of like a complicated feedback linear regulator.

>Bypassing the emitter would give a little extra negative swing because the emitter would not be rising up to meet the collector part way.



This is good. I don't want the driver to cause the clipping either way. My preamp transistor did clip, but I cared little for that because the driver transistor clipped harder.

>Bootstrapping is positive feedback so you do have to be careful with it. Too much and you've got an oscillator.



I might try to design without this feature, but I'll have to at least try it to see how I like it. Why don't you see that happen often in tube amplifiers?

>Gain calculations can be difficult because they require knowing the AC parameters of the transistor.



I guess I'll just have to put it together and see what happens.

>Eliminating the 1 ohm resistor is unwise. Transistors can always run away especially in this configuration.



Good to know. I might put two half-ohm resistors in each emitter lead. This will help self-bias the output transistors just a little. On a more powerful amp (twenty or more watts) I would want to omit these and put the runaway prevention into the base circuit. Resistors like that tend to chew up output power.

>The artifact you are seeing is probably caused by the rubber diode unloading the collector just before the voltage gets to the positive rail.



I'm not sure what you mean by that but I'm sure it will make sense soon.



Then, after all that, Mike wrote:

>I'm more accustomed to seeing that 1 ohm resistor split between

the two emitters.



So am I. I just put it there so I could keep an eye on the power-stage current. I can expect that arrangement to change once I get into a final plan.

>You've got a collector current of about 7 mA and a collector voltage of about 5.1 V. You've got a Vce of about 4.4 V. If you cut the transistor completely off, then the upper peak would be 12.2 V.



I've learned that the 1/2 VCC point for the collector isn't always the best design. When a transistor is loaded, you get a max. signal swing when your Vcq is a little less than what your DC bias would normally be. The difference is usually dictated by the intensity of the AC load resistance. I can't expect my peak output to be 6.1 volts, but 5.5 is good enough. All I really want at this point is higher than 4.

>You may either need a "stiffer" power supply, or you may need

more driver gain, like with Darlington outputs.



The latter may be true. I was running this test using a computer PSU, with the 12v rail rated at 15A. I don't think the power supply was the problem. I calculated the impedance of the driver/power amp at around 11K, so it wouldn't have put a severe load on a 1K output impedance.

>Were these tests conducted with a load on the output, or unloaded?



Yes, I had either an 8 ohm speaker or resistor across the output when testing. I know that funny things happen to gain when you don't load amplifiers properly.

>Have you 'scoped your power supply lines while you are clipping?



Nope.



Ed



----------------------------

#345 Jan 19, 2011

Bootstrapping was used in that big cathode loaded amplifier, was it the

dynaco? The HK A 300 uses positive feedback to boost the overall gain and

make the NFB more effective. The engineers knew what they were doing and

kept the amount of feedback below the level to sustain oscillation.



For oscillation you need a zero phase shift, positive feedback, but you also

need a gain of unity or greater. If the gain is kept less than unity for

all frequencies the effect of positive feedback is to increase the gain.



Regards.



Max. K 4 O D S.



Email: max@...



Transistor site www.funwithtransistors.net

Vacuum tube site: www.funwithtubes.net

Music site: www.maxsmusicplace.com



To subscribe to the fun with transistors group send an email to.

funwithtransistors-subscribe@yahoogroups.com



To subscribe to the fun with tubes group send an email to,

funwithtubes-subscribe@yahoogroups.com

----- Original Message -----

From: "Der_Hellste_Stern" lordof16bit@...>

To: funwithtransistors@yahoogroups.com>

Sent: Tuesday, January 18, 2011 1:05 PM

Subject: [funwithtransistors] Re: More on the class AB transistor amp:





> Hello again,

>

> Max writes:

>

>>Biasing the base of the driver through resistors from its collector adds

>>local negative feedback around that stage. Its major effect will be to

>>stabilize the DC operating point.

>

> Which is good. I hate having to recalculate the entire driver when I want

> to change the value of one resistor. I am hoping that this configuration

> acts like some sort of "autobias". By this term, I mean a self-adjusting

> DC network sort of like a complicated feedback linear regulator.

>

>>Bypassing the emitter would give a little extra negative swing because the

>>emitter would not be rising up to meet the collector part way.

>

> This is good. I don't want the driver to cause the clipping either way. My

> preamp transistor did clip, but I cared little for that because the driver

> transistor clipped harder.

>

>>Bootstrapping is positive feedback so you do have to be careful with it.

>>Too much and you've got an oscillator.

>

> I might try to design without this feature, but I'll have to at least try

> it to see how I like it. Why don't you see that happen often in tube

> amplifiers?

>

>>Gain calculations can be difficult because they require knowing the AC

>>parameters of the transistor.

>

> I guess I'll just have to put it together and see what happens.

>

>>Eliminating the 1 ohm resistor is unwise. Transistors can always run away

>>especially in this configuration.

>

> Good to know. I might put two half-ohm resistors in each emitter lead.

> This will help self-bias the output transistors just a little. On a more

> powerful amp (twenty or more watts) I would want to omit these and put the

> runaway prevention into the base circuit. Resistors like that tend to chew

> up output power.

>

>>The artifact you are seeing is probably caused by the rubber diode

>>unloading the collector just before the voltage gets to the positive rail.

>

> I'm not sure what you mean by that but I'm sure it will make sense soon.

>

> Then, after all that, Mike wrote:

>

>>I'm more accustomed to seeing that 1 ohm resistor split between

> the two emitters.

>

> So am I. I just put it there so I could keep an eye on the power-stage

> current. I can expect that arrangement to change once I get into a final

> plan.

>

>>You've got a collector current of about 7 mA and a collector voltage of

>>about 5.1 V. You've got a Vce of about 4.4 V. If you cut the transistor

>>completely off, then the upper peak would be 12.2 V.

>

> I've learned that the 1/2 VCC point for the collector isn't always the

> best design. When a transistor is loaded, you get a max. signal swing when

> your Vcq is a little less than what your DC bias would normally be. The

> difference is usually dictated by the intensity of the AC load resistance.

> I can't expect my peak output to be 6.1 volts, but 5.5 is good enough. All

> I really want at this point is higher than 4.

>

>>You may either need a "stiffer" power supply, or you may need

> more driver gain, like with Darlington outputs.

>

> The latter may be true. I was running this test using a computer PSU, with

> the 12v rail rated at 15A. I don't think the power supply was the problem.

> I calculated the impedance of the driver/power amp at around 11K, so it

> wouldn't have put a severe load on a 1K output impedance.

>

>>Were these tests conducted with a load on the output, or unloaded?

>

> Yes, I had either an 8 ohm speaker or resistor across the output when

> testing. I know that funny things happen to gain when you don't load

> amplifiers properly.

>

>>Have you 'scoped your power supply lines while you are clipping?

>

> Nope.

>

> Ed

>

>

>

---------------

>

> Always keep electrons and holes flowing in opposite directions.Yahoo!

> Groups Links

>

>

>

>







Contact Us
This Site's Privacy Policy
Google's privacy policies

S
e
n
i
o
r
T
u
b
e
.
o
r
g