Improving Gain and Adjustments of Op Amp Emitter follower

Thread Starter

Sir Kit

Joined Feb 29, 2012
188
Please refer to attached LTSpice screenshot below. It is a single ended op amp emitter follower intended to accept audio line output.

The amp works as in the simulation, except maximum amplitude out is 4.7Vpp and not approx 10V as in the plot. Not having an excellent LT1006, I substituted an LM358. Am I correct in thinking the voltage shortfall is because the LM358 is not rail-to-rail, or is there another reason?

As drawn, the offset and gain are interactive. This makes adjustment fiddly. What is the simplest way to modify the circuit so the gain adjustment does not affect the offset and vice versa? I would prefer to keep the amp direct coupled, i.e. no caps in the signal path. Note: R11 and R9 are nominal values for each leg of the 10K pot.

Any other general suggestions on improving my circuit would also be greatly appreciated.

ltspice-op-amp.png
Test points are a op amp pin 3 (green), 2n222 base (blue) and 2N3055 emitter (red).
 

Ian0

Joined Aug 7, 2020
10,005
You should get about 9V p/p from a LM358 - it gets within about 1.5V of the top rail, and you lose another 1.2V in the transistors. What does the output waveform look like at the same points as in your simulation?
If you get an op-amp with low offset voltage, and use 1% resistors you should be able to achieve the desired output with no need for adjustment.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
188
Thank you for your reply. The output is a sine wave. There is little or no DC offset. Being a retired hobbyist, I would like to be relatively sure of the solution before special ordering another op amp.
 

Ian0

Joined Aug 7, 2020
10,005
If it's a decent looking sinewave, just the wrong amplitude, my guess is that one of your resistors is the wrong value.
Check if the voltages at the op-amp inputs are consistent with the resistors that you used.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
188
As requested, here is the .asc file.

Tomorrow I will try increasing the gain trimmer to 100K, but 50K works in LTSpice.

Any suggestions on how to fix the interactive offset and gain issue mentioned in my OP?
 

Attachments

Thread Starter

Sir Kit

Joined Feb 29, 2012
188
Thanks for the added suggestions. I would prefer to have no cap in the signal path though. Some of the intended frequencies are sub audio. I will try it both ways and report back tomorrow. I also found a rail-to-rail TLV272 to replace the LM358.
 

LowQCab

Joined Nov 6, 2012
4,196
Substituting a Resistor for an Inductive-Coil is certain to
not give the same results in the real-world-Circuit.

It would help tremendously if You would explain why You are building this project
and what your specific expectations are as far as performance.

There may be vastly superior, and/or, more simple methods of achieving what You want.
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Thread Starter

Sir Kit

Joined Feb 29, 2012
188
Thanks for making that point. But the particular coils we are using present a resistive load. They are air core and have no significant inductance.
 

Ian0

Joined Aug 7, 2020
10,005
Thanks for making that point. But the particular coils we are using present a resistive load. They are air core and have no significant inductance.
So why not simply use a resistor? And knowing that the only purpose of the device is to dissipate heat, who cares about the offset?
 

LowQCab

Joined Nov 6, 2012
4,196
Thanks for making that point. But the particular coils we are using present a resistive load. They are air core and have no significant inductance.
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That is, until your Circuit turns into an out of control Oscillator .........

Why are You building this Project ?
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Thread Starter

Sir Kit

Joined Feb 29, 2012
188
So you think air core coils have no useful application? It does not matter anyway. I have already asked the questions that I was hoping to be answered. Based upon the helpful responses so far I will continue in the morning.
 

LowQCab

Joined Nov 6, 2012
4,196
"" So you think air core coils have no useful application? ""
I don't understand how You came up with this question.

And, why do You want to keep everybody guessing as to what this Circuit is supposed to do.
A forum is supposed to benefit everyone who participates in it,
or happens to come across it in a search,
it's not here just for Your personal benefit.
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