Ara Ghazaryan, I would just buy a
LM675 from
Digikey for US$5.50, or the
others shown on this page.. Set it up with a gain of ~5 operating on split supplies of +25 and -25V to +30V and -30V (see Fig 2 on the data sheet). Put it on a 3W heatsink. Done...
View attachment 98094
Hi Mike,
I had a look at Ara's requirement and think that a linear amplifier with DC coupling would give the best performance. After trying a bridge circuit with a single 24V power supply line and input level shifting, I came to the same conclusion as you, that +24V and -24V supply rails would be by far the best approach and would be the simplest to implement, although I had missed the significance of your post before I did a circuit.
At first, I was going to use the OPA549, but dropped that idea when I found it was £15 UK. Then I moved to the OPA548, but even that was £10 UK. But the LM675, remarkably, is only £3.77 UK- a good recommendation. I wouldn't like to give the impression that I am hijacking your idea but here is a practical circuit that should, as you say, do Ara's job nicely.
Ara, if you would prefer to build your own discrete linear power opamp in place of the LM675 I can post a suitable circuit. That approach would involve more work and cost though. All the same, you may like to go the discrete route just for the experience.
spec
ERRATA
(1) Title along lower of the schematic should read Iss01.00
(2) Connect 1N400x rectifier diode cathode to OUTPUT and anode to -24V SUPPLY LINE (x= any number between 2 and 9) (negative catching diode or snubber to stop inductive voltages from the coil damaging the ouput transistors inside the opamp.
(3) Connect 1N400x rectifier diode anode to OUTPUT and cathode to 24V SUPPLY LINE (+ve catching diode)
NOTES
(1) The voltage gain of the amplifier at the wiper of RV1 is 1+ (R2/R1)= 23
(2) The physical layout of amplifiers is vital to ensure frequency stability and low distortion. That is why the earth connections probably look a bit strange.
(3) For the same reasons as (2) above good decoupling is essential. The 100nf decoupling capacitors especially should be mounted as close to the opamp supply pins as possible and the capacitor leads should be as short as possible.
(4) 1mF = 1,000uF in two instances (m= mili, u= micro)
(5) R3 has no direct circuit function. It is just there to give some isolation between the opamp input and the parasitic capacitances and inductance liable to be on the input. R3 should be mounted as close as possible to the opamp input pin.
(6) N1 should be mounted on a substantial heat sink. An aluminum equipment case can often make a suitable heat sink.
(7) The LM 675 is both short circuit and thermally protected by internal circuits. So it won't be damaged if it gets too hot. But what it will do is limit the output current until it cools down. This can be very misleading.
(8) You can also use the circuit as a general purpose power amplifier in servo loops for example. It will also drive any loudspeaker of 4 Ohms or more and should give quite good quality sound.
(9) RV1 is a 1K Ohm linear potentiometer. The apparently low resistance value is to minimize input offset voltage effects and better match your signal generator output (I think). For audio work RV1 should be changed to a 10K Ohm logarithmic potentiometer if you want to use it as a volume control. Also for audio work, it would be best to put a 470nF or larger polycarbonate (or less good a polyester) capacitor in series with the top of RV1.
(10) C6 and R5 form a rudimentary Zobel network to correct some of the phase changes introduced by a reactive load.
DATA SHEETS
(1) LM675 Power Opamp
https://www.ti.com/lit/ds/symlink/lm675.pdf