Project: CMV2A virtual ground circuit

Thread Starter

bloguetronica

Joined Apr 27, 2007
1,424
This simple circuit works as a substitute for a more expensive dual tracking power supply. It is meant for lab or experimental use as is practically fool-proof: it can withstand shorts from ground to one of the rails, polarity reversals and any kind of difficult load, inductive or capacitive. However, it should be used with a single regulated power supply and the current should be limited to no more than 5A, since it has no protection fuses built in.

The circuit is pretty straightforward. It uses an OPA548 as a voltage follower (in a particular configuration to avoid instability if the output is shorted, as voltages at the inputs should always be within the CMVR ). Resistors R2 and R3 work as a voltage divider, which in turn yields the reference voltage for the circuit (C4 and C5 are for noise suppression). R1 defines the current limit value to around 2A (again, C3 is for noise suppression). C6 plays an important role, as it cancels any noise present in the feedback network (without C6, R4 and R5 offer a noise gain greater than unity, even if, in practical terms, the overall gain is still unity). R6 and C7 constitute a Zobel network to add more stability. Diode D1 protects against reversed polarity. On the other side, D2 and D3 protects the output against high voltage spikes that might be generated by inductive loads and also helps dealing with highly capacitive loads, by providing a discharge path.

List of components:
C1 – 100µF electrolytic capacitor (50V);
C2/6/7 – 100nF polyester capacitor (63V);
C3-5 – 10nF polyester capacitor (63V);
D1 – SB550 Schottky rectifier diode;
D2/3 – SB350 Schottky rectifier diode;
HS – 1,2°C/W passive heatsink;
IC – OPA548 power operational amplifier;
J1/3 – Red banana jack (4mm);
J2/5 – Blue banana jack (4mm);
J4 – Black banana jack (4mm);
R1 – 21,5KR metal film resistor (±1% tol., 1/8W);
R2/3 – 10KR metal film resistor (±1% tol., 1/8W);
R4 – 10KR carbon resistor (5% tol., 1/8W);
R5 – 56KR carbon resistor (5% tol., 1/8W);
R6 – 10R carbon resistor (5% tol., 1/2W).

Electrical parameters:
- Vd. mín.= 8V
- Vd. máx.= 32V
- IL máx.= 5A
- Id.(Vd. = 8V, IL= 0A) = 17,4mA
- Id.(Vd. = 32V, IL= 0A) = 18,6mA

Operating characteristics:
- Current limiting (to ground): ±2,021A
- Noise: < 0,2mVrms
 

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Thread Starter

bloguetronica

Joined Apr 27, 2007
1,424
not bad , quite a big board and heatsink :)
Adding to Bill's answer, the heatsink has to dissipate 32W in the worst case. This circuits acts as a voltage regulator with a twist: the OPA548 is able to either source or sink current due to its push-pull output configuration. It uses a configuration similar to the one suggested in the LM675 datasheet, with different resistors and with additional capacitors to suppress noise.

...
Nice Job.
...
Thanks, Bill!
 

Thread Starter

bloguetronica

Joined Apr 27, 2007
1,424
Although this project was mainly designed to power op-amp circuits, it can also power difficult and demanding loads, such as power amplifiers. In the following video, you can see it powering an LM3876 based power amplifier. The amplifier was producing around 15W (given the ±15V supply and the 4Ω load).

See the video here: http://vimeo.com/74863188
 
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