Op Amp to Push Pull Emitter Follower

Thread Starter

Xerxsea

Joined Feb 23, 2022
8
Hey so I have been trying to learn about amplifiers and I've been trying to design one with an op amp IC that I have from a component kit I have. I think i designed the op amp right, taking a 12v vcc and a 0.05v peak input it has a peak output voltage of 5v (10v PtP) however i tried to build a push pull output stage for it and my voltage gain dropped to around half of what it formerly was and I am not sure why. Perhaps somebody much more knowledgeable can help me understand and point out any errors in my design. It seems to me it is an issue with connecting the two stages, i tried to move the feedback loop to be from the output back into the opamp as I had seen done online but I couldn't get that to work properly.

I am also unsure of what exactly the purpose of C5 is as it doesn't seem to change anything if I don't have it, but I saw it as part of many other schematics.

Note: I'm a hobbyist just trying to learn this in my free time I don't have any formal education on the matter

Thanks in advance for any help!

Specs:
Vcc: 12v
small signal input: 0.05v
unconnected opamp output: 5v Peak
Power Stage output 2.8v Peak

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crutschow

Joined Mar 14, 2008
34,457
i tried to build a push pull output stage for it and my voltage gain dropped to around half of what it formerly was
The op amp output current is being limited at the peak.
See the op amp current below (top traces).
I increased the values of R5 and R6, which increased the clipping point

I also changed diodes D1 and D2 to standard silicon junction diodes to reduce the noticeable crossover distortion.
i tried to move the feedback loop to be from the output back into the opamp as I had seen done online but I couldn't get that to work properly.
See below.
I removed the C3 coupling capacitor, so there can be DC feedback from output to input to stabilize the output DC operating point.
It also provides AC negative feedback from the output to reduce distortion.

(Why do you have so much AC gain?
The input to an audio amp is normally a few hundred mVrms.)
what exactly the purpose of C5 is
It's to rolloff the high frequency response which could cause instability or oscillations otherwise.
It may not be needed for this particular circuit, since there is no added gain at the op amp output.

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Thread Starter

Xerxsea

Joined Feb 23, 2022
8
The input is so little as I'm trying to replace a certain amp circuit that is pretty old, it takes an 8 ohm speaker as a microphone input (old intercom system) it produces very little input voltage from my measurements 50mv might even be too high.

maybe I need to add another op amp stage if I need more voltage gain there.

i made the changes you recommended and turned the gain down a bit on the op amp and im no longer getting the clipping on the output current of the op amp and get around 3.5v output peak now which is a bit higher, but should I not be expecting unity gain (~1x) out of the push pull stage? i have as much as 8v peak after the op amp stage, how can I make it so I get that same amount out of the push pull (Or nearly that much)?

Thank you for your help I would have never known take out the coupling capacitor between the two stages. or to check the current out of the op amp.
 

Ron H

Joined Apr 14, 2005
7,063
Q1 emitter and collector are swapped. It still works, but the inverse beta (current gain) will be low. In your sim, look at base currents on Q1 and Q2. They should be fairly close in magnitude when they are properly installed.
 

Ron H

Joined Apr 14, 2005
7,063
I'm guessing you don't want to add 4 more (small signal) transistors, but if you replace R5 and R6 with couple of 6ma current sources, you op amp will be loafing. You really don't gain much headroom, though.audio amp test with cc.jpg
 

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Thread Starter

Xerxsea

Joined Feb 23, 2022
8
Oh ha! totally missed that, Thank you for the correction on Q1 direction as well! seems like 4v peak, 8v Ptp is about the max i can get out of that op amp before the output starts clipping, is that normal for a 12v single power supply? Is there a spec or some rule I'm missing here? I thought I could get a lot closer to 6v peak (somewhere around 5), Like I can when the op amp has no load. I think this might just be due to not using a +- supply configuration? around 70% of rails is not too bad I suppose

I'm sorry I am unfamiliar with the term loafing. what does that mean?

I will try to build the circuit now I guess! Thank you for the help everybody!
 

Ron H

Joined Apr 14, 2005
7,063
Oh ha! totally missed that, Thank you for the correction on Q1 direction as well! seems like 4v peak, 8v Ptp is about the max i can get out of that op amp before the output starts clipping, is that normal for a 12v single power supply? Is there a spec or some rule I'm missing here? I thought I could get a lot closer to 6v peak (somewhere around 5), Like I can when the op amp has no load. I think this might just be due to not using a +- supply configuration? around 70% of rails is not too bad I suppose

I'm sorry I am unfamiliar with the term loafing. what does that mean?

I will try to build the circuit now I guess! Thank you for the help everybody!
Loafing means not working hard.
Regarding output range, the op amp isn't great in that regard. look at the datasheet.
 

Thread Starter

Xerxsea

Joined Feb 23, 2022
8
Thank you, just wasn't entirely sure what I'm looking for in the data sheet, I saw it had a closed loop gain of up to 40db at my operating frequencies, so I thought it was satisfactory. I see another graph of Output voltage vs Load resistance, which would be just the one 300 ohm resistor as per the modified crutshow diagram? Shows a voltage of 9v positive swing at 300ohm giving me the 4.5v peak (9vPtP) you were able to acquire with the current source set up? Is this spec the limiting factor then? Am I understanding this correctly?

Thank you again for your time and knowledge!
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LowQCab

Joined Nov 6, 2012
4,075
If you're trying to learn more about Transistors, the above scenario is a great way to do it,
but if You are looking for maximum-Output with only a single ~12-Volt-Supply,
You would be much better-off using a 2-Channel Audio-Amplifier-Chip wired for a Bridged-Output.
This will easily provide more than twice the Output-Power, with less Distortion, and it's very simple to put together.
An Aluminum-Box or other adequate Heat-Sink will be required in any case.
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