OpAmp low output current to high current across load

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engr_david_ee

Joined Mar 10, 2023
166
I am wondering if I have an OpAmp which can deliver maximum output current for example 50 mA and the output voltage across the load is -3 V and I need around 200 mA load current with -3 V. I found an approach to get higher output current then OpAmp. This can be done using two transistors but I am not able to understand how it work. Can someone please describe how the two transistors in the attached circuit work to get higher current ?
 

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crutschow

Joined Mar 14, 2008
34,455
The transistors act as push-pull emitter followers.
If you only need negative output voltage and current, then you just need the PNP transistor.

What power supplies do you have?

Do you understand emitter-follower circuit operation?
 

dl324

Joined Mar 30, 2015
16,930
Can someone please describe how the two transistors in the attached circuit work to get higher current ?
The transistors act as current amplifiers. What you've shown is a class AB amplifier. You don't need that because your desired voltage in negative.

All you need is a voltage follower.
 

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
Thanks for comments. I understand the concept of cross over distortion in push pull amplifier i.e., if the input to the base of the PNP and NPN in push pull amplifier is between +0.7 V and -0.7 V, none of the transistor will conduct.

I would like to explain bit more the actual problem in connection to post #1.

We have a DAC that is driven by an FPGA. The DAC is outputting variable 0 V to 3 V. We need to invert it. We already are using an op-amp in inverting configuration with unity gain that invert the polarity and gives 0 V to -3 V at the output of an op-amp. The op-amp we already have selected and for some reason we can not change the op-amp. The problem is that the op-amp is rated up to 50 mA output current and we need 200 mA through the load with variable negative voltage 0 V to -3 V.

The output of the op-amp will never be positive because we invert the output of the DAC using op-amp in inverting configuration. The output of the op-amp will be varing between 0 V to -3 V.

I was thinking to use push pull amplifier stage using PNP and NPN at the output of the op-amp as shown in the post #1. I guess in order to overcome the crossover distortion there are two solutions.

1- I can permanently bias both the base of transistors PNP and NPN using resistors and diodes so that both transistor always conduct.
2- Drive DAC to get 0.7 V to 3.7 V instead. The output of the inverted op-amp will be from -0.7 V to -3.7 V. And the push pull stage will give 0 V to -3 V.

Kindly suggest which option is better.

A question about push pull amplifier. In worst case, if the input signal is between +0.7 V and -0.7 V, and there is no diodes to turn the transistors on all the time. This is the case when none of the transistor will conduct. What will be the output when none of the transistor conduct ?. In our case, we can accept 0 V at the load which is not a problem.
 

crutschow

Joined Mar 14, 2008
34,455
The output of the op-amp will never be positive
Then you only need the PNP as the NPN will never conduct.
I guess in order to overcome the crossover distortion there are two solutions.
There is no crossover distortion if the output is only negative.
if the input signal is between +0.7 V and -0.7 V, and there is no diodes to turn the transistors on all the time. This is the case when none of the transistor will conduct.
Not true, since the feedback is from the emitter output, so the high open-loop gain of the op amp will generate voltage to turn one or the other transistor on for just a few microvolts of input change.
Crossover distortion is generated during the time it takes the op amp output to slew the 1.4V difference between turning off one transistor and turning the other on.
Due to this fixed slew-rate time, the distortion gets worse as the frequency is increased.
 

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
In our case the input signal is changing is very very slow. Most of the time we need a stable output but at variable levels controlled by the DAC slowly changing.
 

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
Alright. I understand that I only need one PNP transistor at the output of op-amp to get current around 200 mA because the output of op-amp will never go positive.

But do I need a permanent bias -0.7 V using diode or voltage divider to turn ON BE junction of the PNP all the times ?

What will be the emitter output of the PNP transistor when the output of the op-amp is between 0 V and -0.7 V. During this period, the BE junction will not conduct and the PNP transistor will be OFF.
 

crutschow

Joined Mar 14, 2008
34,455
What will be the emitter output of the PNP transistor when the output of the op-amp is between 0 V and -0.7 V. During this period, the BE junction will not conduct and the PNP transistor will be OFF.
No.
Did you not read my previous explanation?
Due to the feedback from the transistor's emitter, and the op amps high open-loop gain, that dead spot will never occur.
 

Thread Starter

engr_david_ee

Joined Mar 10, 2023
166
No.
Did you not read my previous explanation?
Due to the feedback from the transistor's emitter, and the op amps high open-loop gain, that dead spot will never occur.
This point is very important for me to understand. I am wondering if possible to draw for me the feedback from transistor emitter.
 

ronsimpson

Joined Oct 7, 2019
3,037
The circuit in #13 helps deduce crossover distortion.
Here is another way. I added a B-E resistor (in red).
Your op-amp is good for 50mA. So set the resistor so when there is 0.7V across it there is 10mA of current. At very small loads the current from the op-amp will flow through the resistor to the load and Q1,2 will not turn on. If the load current is above 10mA, the first 10mA comes from the op-amp but all the current above that comes from the transistors.

Circuit from #13 has better distortion in audio circuits but #14 is also good but works the op-amp a little harder.
1708813451034.png
 

crutschow

Joined Mar 14, 2008
34,455
I am wondering if possible to draw for me the feedback from transistor emitter.
Here's a sim of the push-pull circuit with feedback from the emitter outputs.
Note that the op amp output (red trace) follows the input (yellow trace) with the transistor ≈0.7V base-emitter offset from the output (green trace) until the transistor shuts off at the op amp output of 0.7V.
This opens the loop and the op amp now slews as fast as it can to the other transistors 0.7V turn-on voltage until it starts conducting.
You can see the crossover-distortion ripple in the output during this transition.

That all make sense?

1708813880560.png
 

sarahMCML

Joined May 11, 2019
370
All you need for your "negative only" output is circuit #15 above, with Q1 removed! Feedback automatically takes care of the 0.7V base/emitter drop, the output of the opamp being always 0.7V more negative than the positive input.
 
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