Simple Bench Amplifier for PC Generated Test Signals

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
I would like to build an amplifier to boost test signals generated in software. Specifically, low frequency sine waves or other basic waveshapes.

The circuit should operate on 12VDC, connect to one channel of my PC headphone jack, and output approx 12Vpp of direct coupled unidirectional current at around 2A. The intended loads are non-inductive.

The likelihood that some of the signals will be subaudio, and the requirement for unidirectional current, rules out any type of stock audio amp. I just need something simple and robust I can build into a black box for regular reliable use.

This old standby is what I am considering, with10uF, 2K2 and TP41/42, but any suggestions for improvement would be geatly appreciated. The sound card output will be bipolar, but I am assuming this will be removed by the input caps.

class_ab.gif
 

BobTPH

Joined Jun 5, 2013
4,772
That circuit needs either a bipolar supply or a coupling capacitor on the output. And it has no voltage gain.

Bob
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
Thank you for pointing that out. I could possibly add a class A driver as below.

class_a_driver.gif R1 22K, R2 10K, RL 22R, RE SOT, C1 2.2uF, C2 22uF

Or an op amp with fixed 10x gain LM358, etc. Or a power op amp such as the LM675.

Which of the three would be "better" for my generic bench-type application?

Is there any way to dispense with the output coupling cap and still use a single supply? I am concerned it may affect any subaudio signals.
 

LowQCab

Joined Nov 6, 2012
1,931
You need to explain exactly what You are trying to accomplish,
not what Circuit, or Amplifier type, that You are thinking about trying.

Your request is not understandable,
You can't have an Amplifier with a 12-Volt "Peak to Peak" Output that's powered by 12-Volts.
12-Volts "Peak to Peak" requires in excess of ~24-Volts, and is certainly not "uni-directional".

What is "Direct-Coupled" "unidirectional" Current ?
It sounds like You want a DC-Output,
but Your Computer only supplies an AC Signal.
.
.
.
 

BobTPH

Joined Jun 5, 2013
4,772
I think he wants a sine wave with DC offset equal to the peak voltage. In this case a 6V peak sine with 6V DC offset, so varying between 0V and 12V.

Didn’t we already di a kong thread about this?

Bob
 

BobTPH

Joined Jun 5, 2013
4,772
What current do you need at the output? An opamp can easily do this, and there are high power opamps if you need more than the 20-30 mA a normal opamp can output.

Bob
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
To address the points raised.

I would like a single rail 12VDC powered amp with as much voltage out as possible.

Op amp best solution? I did mention the LM675. But I believe the biasing required at the input to produce the 6V offset at output will change with the peak input voltage. If possible I would prefer not to make this adjustment with each use.

Input voltage is the normal range provided by sound card out. About 2V max. Output as close to rail as possible, given fixed gain at actual input volts. Current up to 2A.

Thank you for asking.
 
Last edited:

ronsimpson

Joined Oct 7, 2019
2,012
If all you have is +12V and you want a 0 to 12V signal, there is a problem. You need a R-R output amplifier.
I would use a audio amplifier. No audio amp (that I know about) is R-R output.
1643489895620.png
I looked for a 2A R-R output amp and found this one. But it will not work at 12V.
1643489979017.png
What are you driving with this signal?
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
As close to 0-12V swing as I can get. Not exact. That amp IC is nice but would be push-pull out I believe, and low voltage as you mention.

I will probably need to build something and therefore looking for the simplest workable option within the specs already given. Maybe just an LM358 with NPN emitter follower similar to this. Comments?

12v_amp.png
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
Right. LM675 minimum 16V supply. I seem to recall running one on 12V. Will need to check.

How could I fix the pull-down in my attached op amp circuit?

R6 is the "load".
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
Then about 2V total with emitter follower. Look on the bright side.

Based upon the responses so far, I think will go with the single supply op and and follower. Offers gain and least voltage drop with a few basic parts.

Still wondering how to add the "pull down" Ron mentioned.
 
Last edited:

Thread Starter

Sir Kit

Joined Feb 29, 2012
127
Thanks Ron. This inspires confidence. I will build this week. Substitute trusty LM358 for LT part which I do not have on hand. I assume R2 is 1K as well. Very much looking forward to it.
 
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