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Pimping an Op-Amp for wide voltage swings

A wide-voltage swing op-amp for less than £1.00?

As promised in an earlier post, I describe here my experience in turning a common-or-garden TL081 PDIP op-amp into a wide-voltage swinging monster, with a minimal sprinkling of SMD and a small circuit board made from scraps.

Prototype 72 volt op-amp

Prototype 72 volt op-amp

I wanted to augment my current heavy-duty power-supply to use electronic control of the output voltage(s), rather (but as well as) than the 500watt VARIAC I use at present. The full output voltage from this unit is around 45 volts, but I anticipate the maximum I need to supply using electronic control is around 35 volts. This fits in well with the use of the standard positive and negative regulator ICs LM317 and LM337, with suitable current boost. In a previous post I demonstrated the use of a simple op-amp to supply the negative side voltage drive, enabling the use of a single pot for full dual-tracking. (Simple Dual-voltage Tracking Power Supply for 3 to 15 volts) Unfortunately, the TL071/81 op-amp I used is limited, amongst many others, to an absolute maximum supply voltage of +/- 18volts. Clearly, one solution is to substitute a high-voltage op-amp in place of the TL071/81, but two factors mitigate against this, the first being cost, and the second availability. A browse through suppliers catalogues suggest suitable units are priced above £7.00, (GBP) but the units are not available using local suppliers, (here in the NE of England) so postage & package costs must also be factored in.

Not happy with the somewhat expensive unit cost, I looked for alternatives, and came across an article published in May 1999 in EDN magazine, describing a solution using a standard op-amp and a few discrete components. The paper is worth reading, and I have linked a copy here on my site: http://joebrown.org.uk/images/DualPSU/BootstrappingOpAmps.pdf in case you can’t find the article elsewhere.

The article describes and discusses the method used, and I’ve included the schematic below.

Original bootstrap schematic from EDN, May 1999.

Original bootstrap schematic from EDN, May 1999.

If you intend duplicating my experience, I strongly urge you to read the article in full, or at least take particular notice of the paragraph ‘DC Conditions
as this has implications on which configuration you can employ the op-amp in.

I drew a generalised form of the above schematic in Eagle, and produced small circuit boards for both SMD and DIL-packaged TL071/81 op-amps. With single-sided configurations, no real saving in space is achieved by using the SMD version of the op-amp, so I present only the DIL-packaged version. The Eagle project is provided, for those wishing to experiment further. This is a practical circuit, and intended to run with power supply lines of no more than +/- 36volts. Note my comments on the schematic WRT to the transistors used, and the values of R3 and R4, if higher voltage swings are required.

Wide voltage swing op-amp prototype schematic

Wide voltage swing op-amp prototype schematic

Component Choice.

I opted for SMD for all of the discrete components, and although the original article suggests using the base-emitter diode of another two transistors for D1 and D2, I used a couple of MINIMELF-packaged 1N4148s. R3 and R4 were reduced in value to 10K, so as to place the op-amp supply near it’s maximum. Suitable resistors should be substituted if you intend using an op-amp with a different maximum supply voltage.
I fitted a 330pF capacitor across the bases of the transistors. This effectively killed a high-frequency parasitic oscillation evident as the supply voltages were increased during testing.

Construction.
I opted to mount all components, including the DIL-socketed Op-Amp, on the foil side of the board, this keeps the side profile as narrow as possible, as there is plenty of space underneath the turned-pin socket I used for the SMD transistors etc. If you wish to mount the Op-Amp on the ‘top’ of the board, then by all means mirror what I have produced in Eagle.

Testing.
The sub-assembly was plugged into a bread-board and set up as an inverting amplifier with a feedback resistor Rfb of 100K, and an input resistor Rin of 15K. The + (non-inverting) terminal of the unit was taken to ground. Note that drive current is limited, so low load resistances will limit the peak swings. The oscillograms below were captured with the op-amp driving only the oscilloscope probe. Practical load resistances below around 15K will start limiting the output before clipping occurs.

Oscillogram of input/output with supply voltages +/- 35.5volts

Oscillogram of input/output with supply voltages +/- 35.5volts

Oscillogram of Output and VCO with supply voltages of +/- 35.5 volts

Oscillogram of Output and VCO with supply voltages of +/- 35.5 volts

Oscillogram of output and VEO using supply voltages of +/- 35.5 volts

Oscillogram of output and VEO using supply voltages of +/- 35.5 volts

I’ve embedded a Shockwave viewer to my photo collection for this post below, or you can go direct to the site here: http://cullercoats.joebrown.org.uk/#42

http://cullercoats.joebrown.org.uk/#42

Downloads

The Eagle schematic is here: http://joebrown.org.uk/images/DualPSU/HV_OpAmp_Discr.sch
and the board here: http://joebrown.org.uk/images/DualPSU/HV_OpAmp_Discr.brd
High definition exports of the above are available in the photo collection: http://cullercoats.joebrown.org.uk/#42

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