How to analyse a class B push-pull amplifier

Thread Starter

Kelko

Joined Oct 1, 2020
33
Hi, so on my current project we have to analyse a push-pull amplifier but because of covid a lot of material was skipped and left for us to look up and i have no idea where to start. This is the current circuit and I'm not too sure what the resistors R2, R4 and R7 do or what the capacitor c1 is for. Any help explaining it or showing me somewhere i could find a good explanation would be greatly appreciated
1607698117353.png
 

Ian0

Joined Aug 7, 2020
2,614
A few things that need sorting out first:
1) The TIP127 is back to front - emitter goes to the positive supply
2) Where's ground? Some parts are connected to ground and shown by the earth symbol, but no part of the power supply is connected to ground.
3) There's no DC feedback from the final output.
 

Audioguru again

Joined Oct 21, 2019
3,309
With only a 9V supply, the voltage losses in that horrible circuit will result in an output of only 0.27W. It is supposed to have an additional -9V supply with the junction of the batteries at the circuit's ground. Then the power will be 3.2W.
The circuit is on a few websites in India.
Either the TIP122 or the TIP127 is backwards. Both ways result in massive uncontrolled shoot-through current.
The output has no negative feedback which will result in distortion.
The output coupling capacitor values are so small that a man's voice will sound like a chipmunk and music will have no bass.
 

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Ian0

Joined Aug 7, 2020
2,614
I have made this circuit, and it does work and it's perhaps not so horrible. My application was a headphone amplifier, so shoot-through wasn't a problem because it was already class A.
I think R4 should go to ground, C2 should be deleted, and the feedback should be taken from the collectors of the darlingtons. Definitely the TIP127 that's backwards.
With a proper + and - supply the output capacitor can also be deleted.
Perhaps some resistors in the emitters of the darlingtons and it might just stand a fighting chance of the bias current remaining stable, although it could really do with temperature compensation.
I have seen it in a manufacturer's application note somewhere - can't remember where.
You can't make a stereo version with a dual op-amp!
 

Ian0

Joined Aug 7, 2020
2,614
Remembered where I first saw it:
https://worldradiohistory.com/Archive-Electronics-Today/ETI-Electronic-Circuit-Design-No-2.pdf
see page 6.
At a guess, it came from a 741 application note, and it's only purpose was to extract something better than the rather miserable 0.5V/us slew rate out of the 741. I built it because the output could go rail-to-rail (another thing that no op-amp did in the days when it was written).
The 4700pF cap is a form of current feedback - another ahead-of-its-time feature.
I've also seen it with a preset between pins 4 and 7 as a way of adjusting the bias. (What it really needs is a PTC thermistor)
I think we'll agree that it's a circuit that has passed it's "use by" date!
 
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Audioguru again

Joined Oct 21, 2019
3,309
The idle current of the LF351 is 3.4mA max. The current gain of the TIP122 and TIP127 are 5000 max. The Vbe of the darlingtons is about 2V at high currents then the 2k resistors will have 2V across them for 1mA and the bases of the darlingtons will have the remaining 2.4mA in them.
Don't you think the 2.4mA will be amplified by the current gain of 5000 times and produce a shoot-through current of 12A(!)?

You might be lucky to find an opamp with a 1mA idle current Then the voltage across the 2k resistors will be only 0.5V and the darlingtons will not turn on.

It is a hopeless circuit.
 

Thread Starter

Kelko

Joined Oct 1, 2020
33
yeah from everything everyone has said the circuit just seems useless and idk why he chose this one to give us to go off of. All that we were given was that it was used to drive a low impedance speaker of around 8 ohms to 50 ohms. What would you recommend changing about it?
 

Ian0

Joined Aug 7, 2020
2,614
The version on page 6 of
https://worldradiohistory.com/Archive-Electronics-Today/ETI-Electronic-Circuit-Design-No-2.pdf
does actually work, (I know because I built it many years ago) and has a good explanation of how it functions.

But, as @Audioguru again says, it's not a great circuit, and I wouldn't build it again.

To get it going (as good as it's going to get):
Two 9V power supplies to give +/- 9V, with the centre earthed.
Get Q2 the right way round.
Connect the feedback to the output.
Change R3 and R5 to 680 ohms and hope that your particular LF351 is "typical" otherwise it will blow up your transistors

TO answer your original question, R2 and R7 set the gain. C1 reduces the gain at high frequency.
Don't know about R4
 

Audioguru again

Joined Oct 21, 2019
3,309
Is your teacher very old? Did your teacher give you the circuit with the TIP127 upside down and the missing negative supply?

The LF351 was invented by National Semiconductor many years ago. They have not made it for many years.
The TIP line of darlingtons was invented by Texas instruments many years ago. They have not made them for many years.

Most old opamps already have a class-B push-pull output stage, but use tiny low current transistors.
Most IC audio power amplifiers also have a class-B push-pull output stage but they use large high current output transistors and a case with a metal tab to carry away the heat.

You can use a modern rail-rail opamp to drive a class-B push-pull pair of power transistors like this:
 

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Ian0

Joined Aug 7, 2020
2,614
Do you have to analyse it or make it work?
If you teacher got the TIP127 back-to-front, then it's really easy - the analysis is "It won't work because the TIP127 is back to front" - end of analysis!

You might get it to work with the corrections, (have plenty of spare transistor on hand to replace the ones that blow up) but @Audioguru again 's circuit is pretty much guaranteed to work

The LF351 was invented by National Semiconductor many years ago. They have not made it for many years.
The TIP line of darlingtons was invented by Texas instruments many years ago. They have not made them for many years.
LF351 is still made by SGS-Thomson (I think it is identical to a TL071, which Texas does still make, and introduced at the same time that Nat Semi introduced the LF351. Texas took over Nat Semi so no reason to keep making them both).
The Texas TL071 was closer to a Nat Semi LF351 than it was to a SGS-Thomson TL071 or LF351, which had a tendency to instability.
TIP127 is still made by SGS-Thomson and On-Semi.
 
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Thread Starter

Kelko

Joined Oct 1, 2020
33
Do you have to analyse it or make it work?
If you teacher got the TIP127 back-to-front, then it's really easy - the analysis is "It won't work because the TIP127 is back to front" - end of analysis!

You might get it to work with the corrections, (have plenty of spare transistor on hand to replace the ones that blow up) but @Audioguru again 's circuit is pretty much guaranteed to work


LF351 is still made by SGS-Thomson.
TIP127 is still made by SGS-Thomson and On-Semi.
aha i wish it was that easy but yeah I think I'm gonna have to correct it and fix it before analysing it, I'm guessing it he just threw these on here without thinking because honestly he's probably the worst teacher I've ever experienced. The school gave us multisim to work on so thankfully I won't have to deal with buying more transistors. although we haven't covered rail-rail before so could you explain how @Audioguru again 's circuit actually works?
 

bertus

Joined Apr 5, 2008
21,447
Hello,

Here is a copy from the file that @Ian0 showed:
high slew rate amplifier.png
The output current in the 47R resistor determines the driving of the two transistors.

Bertus
 

Audioguru again

Joined Oct 21, 2019
3,309
Did your teacher teach about an emitter-follower? My circuit and many other amplifier circuits use complementary
emitter-follower output stages.

Did your teacher teach about the difference between class-B and class-AB amplifier output stages?
 

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Thread Starter

Kelko

Joined Oct 1, 2020
33
nope we haven't learned about that either, hes basically told us nothing about these type of circuits all we've learned were the basics about different types of opamps and pretty much nothing about transistors
 

Ian0

Joined Aug 7, 2020
2,614
nope we haven't learned about that either, hes basically told us nothing about these type of circuits all we've learned were the basics about different types of opamps and pretty much nothing about transistors
Good job he didn't ask you to analyse an amplifier made entirely out of discrete transistors!

Have a look at the Wikipedia article
https://en.wikipedia.org/wiki/Common_collector
To cut to the chase:
An emitter follower has no voltage gain. For an AC signal the output voltage is almost the same (slightly lower) as the input voltage.
The current is amplified by hFE of the transistor.
The DC voltage is shifted by the Vbe voltage of the transistor. Output is 0.6V lower than input for a NPN transistor, and 0.6V higher for a PNP transistor.

So, for @Audioguru again 's circuit the emitter follower is attached to the output of the op-amp. The output AC voltage would be the same as the AC input voltage. The current would be much greater, but there would be a DC level shift. The top half of the waveform (through the NPN transistor) would be 0.6V lower and the bottom half (through the PNP transistor) would be 0.6V higher.
So, to compensate for the 0.6V shift, we need to add 0.6V to the top half and subtract 0.6V from the bottom half.
That's what the diodes do.

Does that help?
 
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