Yes, R2 would be 1k if you want unity gain.

 

Because there are two schematics glued together there are two different R2s. lol
Gain is set by R2 around the amplifier.
The R2 that pulls down the Bases could be 1K. It is probably not needed if the op-amp can pull down well. (pull down to 0V)

You can look at any 5 to 25 watt audio amp circuit. The output stage is about like this. There are circuits on the internet with op-amp + two transistors. Or 5 transistors and the same output stage. (watch out most circuits on the internet have not been tested and are drawn by clown like me) lol

 

Just so I know, does the current through RL flow in one direction only throughout the entire cycle, or does it reverse direction at the time of each cross-over?

What is the practical advantage of two output transistors over one?

Even if a circuit doesn't work as expected, I always learn something new.

 

I have asked and do not know what the load is. So maybe the bottom transistor is not doing anything. It is typical for the power amplifier to be AC coupled with a big capacitor, just like a audio amplifier. If AC coupled then the bottom transistor does have a job.

 

The load varies. Usually just a resistive air coil of a few ohms to ground. Sometimes a plate capacitor. It is experimental work, not audio listening pleasure.

I would prefer no cap at output as I find it tends to block lower frequencies.

Going back to my question. Assuming I know nothing, what in general is the advantage of a complementary pair over an NPN follower? And, in particular for this application?

If the PNP is included in the circuit, does the current at the load reverse at cross-over?

Thanks again.

 

The load varies. Usually just a resistive air coil of a few ohms to ground. Sometimes a plate capacitor. It is experimental work, not audio listening pleasure.

I would prefer no cap at output as I find it tends to block lower frequencies.

Going back to my question. Assuming I know nothing, what in general is the advantage of a complementary pair over an NPN follower? And, in particular for this application?

If the PNP is included in the circuit, does the current at the load reverse at cross-over?

Thanks again.

Has nothing to do with the presence of a PNP in a complementary pair. The resistor in the NPN follower will do the job, but maybe a bit slower.

 

For testing stuff (including impedance tests on components) I'd want a linear amp with an output referenced to ground, not a bridged output like a "high power" car stereo chipamp or class-D. Things like that can be scrounged from e-waste like old CRT TVs, Ryobi jobsite radios, to name some things I've found discarded. TDA2002 is/was a common chip of that sort, but I was impressed with the performance of the less popular TBA810 since it offered lower distortion, and had a bootstrap capacitor that allows higher peak power for a given supply voltage.
On the other hand, a DC-coupled amp would have some advantages for testing at low frequencies or even DC. The LM3886 might seem like overkill (almost 10 times the wattage rating), but it probably would give a lower impedance output and lower distortion and stuff. Digikey still lists them, but has zero stock. See the "LM3886 availability" discussion at Diyaudio.com for some alternatives.

 

I spent some time on the bench. Still not satisfied with the result. I don't think adapting an audio amp is the answer.

What I need is a general purpose linear "lab" type amp with no caps in the signal path (to allow for near DC input) that will run off +12VDC and provide at least 8Vpp out at 2A. Variable gain 1-20.

It should be able to drive resistive and capacitive loads. To accommodate both mono and bi-polar input signals, there needs to be an offset adjustment.

I don't see any advantage for this application in class AB over A. This type of amp would have lots of uses, and mean I would not have to keep building dedicated ones for each project.

Any further advice, or pointers to a suitable circuit, would be much appreciated.

 

First You need a stout 2-Rail Power-Supply, of about plus/minus 20-Volts.

If you are feeling adventurous ...........
Here's an untested Bridge-Amp I came up with.
If You leave out the Input Capacitor, it should be happy to provide DC Amplification.
If You don't want a Bridge-Amp, just build the left half of the Schematic.
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Here's an untested Bridge-Amp I came up with.

The problem I see is that there is such significant variation in Vgs(th) between MOSFETs of the same type, that the output bias is likely to be seriously off.

 

Here is a circuit that seems to fill most of my requirements. Designed to accept bipolar input from PC sound card.

Expert advice, comments, corrections invited.

Note that the TLC272 is a single supply package.

 

Do You seriously want a Class-A Output-Stage ?
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All current is dumped into resistive load and I believe AB requires caps. So why not?

If you have a better circuit that is single supply please share. Thanks.

 

I used this circuit for the output of my 20 MHz function/sweep generator, but it is low power, using only ±5V power. Maybe you can use part of it.

ircuit Description

The circuit had some apparently tough constraints: High input impedance so as to not load down the output of the MAX038, a low impedance output to drive the coax and any loads, wide bandwidth -as flat as I could get to at least 20 Mhz, and a full bandwidth output voltage swing to within 2.5 volts of the power supply rails.

For the high input impedance the circuit uses a differential pair of 2N3904 transistors. The collector currents are differentially amplified by a 2N3906, which has its 1k collector load connected to - 5 volts though a bias network for the complementary pair buffers the high impedance collector of the second amplifier stage to drive the output termination resistor.

The relatively low values of the resistors are needed to allow sufficient output current to provide the large output range of over 5 volts (- 2.6 volts to + 2.6 volts) while driving terminations, to keep sufficient current through the transistors for them to have adequate bandwidth and so that capacitances don't cause the amplifier stages to roll off at frequencies that are too low.

Once the bandwidth and load driving signal swing were confirmed, the next problem was how to stabilize the amplifier. Having three stages means that at some frequency, there is 180 degrees phase shift between the inverting input and the output. Dominant pole compensation would not work because there was not enough excess gain in the circuit. That left me with the trick of shunting the amplifier's input with a broad band network to reduce the loop gain. That is the purpose of the 0.1 uf capacitor in series with the 100 ohm pot.

The 0.1 uf capacitor and 100 ohm pot is connected between the an input stage's two inputs. The pot is set to about 50 ohms and the impedance of the two 1k resistors that set the closed loop ;gain is 500 ohms, which results in the loop gain being lowered by about 40 db at frequencies above the corner frequency of the 0.1uf capacitor and the 50 ohm resistor (about 30 kHz), which is enough to keep the gain at 80 Mhz, where the amplifier tended to oscillate, less than 0 db. I tried using a fixed resistor, but I noticed that to get excellent square wave response, the loop gain had to be critically adjusted. Using a pot was much easier than selecting a set of resistors.

 

That is a clever circuit, but more than I really need. I am only interested in up to 100KHz and high input impedance is not required. Apart from that, it looks like a fairly standard class AB, similar to what Ron sugested. My class A config will provide all the current my heatsink can handle.

Can anyone please comment on the op amp follower circuit I attached? Drawbacks, corrections, improvements, better alternatives and why?

 

The opamp circuit you posted has the slightly irritating characteristic of the gain being affected by the offset control. Otherwise I don't see any problem with it. It an easy enough circuit to build, and after that you may learn a little more about it.

 

Can anyone please comment on the op amp follower circuit I attached?

Driving a resistive load with a single-supply follower is no problem since the current is only going out, but a capacitive load requires current in both directions and the follower is asymmetrical when doing that, which could give a distorted output.
So it depends upon how large a capacitive load you want to drive.

 

When working with sound card output, I always crank the PC volume up to a full 2Vpp AC before adjusting the offset. Then the gain.

Unfortunately, the existing circuit will not output any more than about 6Vpp. If the gain is increased any further, the top of the waveform is cut off. Can anyone please offer an explanation/solution?

When driving a capacitive load from single supply I normally provide a discharge path via parallel resistance. This can change the wave shape but not significantly at low frequencies.

Once I am happy with the result, I will tweak the component values and repost here. I substituted the TLC272 with an LM358. The former turned out to be duds ordered from overseas.

 

If the gain is increased any further, the top of the waveform is cut off. Can anyone please offer an explanation/solution?

If you are using the LM358, that will cut off the top of the waveform since it's not rail-rail, and the maximum output is a volt or two below the supply voltage.
Edit: You could use a rail-rail type such as the LMC6484A.

 

Why then am I only seeing 6Vpp out with a 12V supply? Even when I increase the supply voltage, it still clips at 6V.

I have checked repeatedly and the circuit is built exactly as per my diagram, shown below.