Electronic Design

Current Mirror Speeds Up Balanced-Output Optoisolator

Circuit and system design requirements often mean compromises. For example, optoisolators are used almost everywhere, but they are highfrequency limited. This simple circuit allows for wider bandwidth at the cost of two transistors and two resistors. And it's all done with mirrors!

With conventional approaches, the resulting design isn't always optimized for high speed. In certain situations, the optoisolator load-resistor value might be higher than the value that provides the best high-frequency response. Changing the load configuration from a common emitter to a common collector doesn't affect performance.

The photons are coupled into the base region and the load connection doesn't affect them. Adding a speedup resistor from the base to emitter isn't always feasible, as some devices don't have an external base terminal. Also, this decreases the effective current-transfer ratio (CTR).

A current mirror offers low input impedance, which is good for high-frequency response, and high output impedance, which is good for voltage gain. In Figure 1, the driver for the diode part of the optoisolator is a differential amplifier made from Darlingtonconnected npn bipolar transistors, Q1 to Q4. Any other driver can be used. This is the one I used for the measurements.

The differential amplifier is biased by a 13-mA current source that's adjustable from 6 to 18 mA, a three-to-one range to accommodate the CTR spread. For simplicity, the schematic shows both circuits: the standard (Box A) and the one with the current mirror (Box B)—one each connected to the two differentialamplifier outputs.

Optoisolator U1 and R3 compose the standard circuit—the one with a simple load resistor. Figure 2, bottom curve, shows the frequency response for this circuit. The –3-dB frequency is 23 kHz. This value falls within the manufacturer's specifications for this circuit under actual operating conditions. Overall voltage gain is 37.7 dB at 1 kHz.

The improved circuit uses U2 and a current mirror made from two pnp transistors, Q5 and Q6, plus two resistors, R4 and R5. The resistors are used for degeneration. They equalize against VBE and other parameter variations between unmatched transistors. Optoisolator U2's output, the transistor collector, feeds the current-mirror input terminal, the base and collector of transistor Q5. The load resistor, R6, connects to the current mirror's output, the collector of Q6. With this connection, we have the same overall input-to-output signal polarity as for the conventional circuit.

Figure 2, top curve, shows the results for this circuit. The bandwidth is improved about 3.5 times to a value of 86 kHz. The voltage gain stays almost the same at 38.1 dB, a 0.4-dB difference. All of this is accomplished without increasing the driver current.

For a complementary output, just use the improved circuit with both optoisolators, U1 and U2. You get complementary signals, isolated from input to output and from one output to the other. Depending on your application, you may need to either match the optoisolators for equal CTR or add a balancing adjustment to the circuit.

But rarely do the system requirements mandate a fully isolated complementary output. If the complementary outputs don't require isolation between each other, use the complementary output circuit shown as Box C, the components around U3. It uses only one optoisolator, but two complementary current mirrors. The current source had to be readjusted at 16 mA to balance the dc between Outputs 3 and 4. Frequency response and gain, both measured differentially, are 45 kHz at –3 dB and 45.8 dB at 1 kHz.

These circuits are low-cost solutions and an alternative to using faster and more expensive optoisolators.

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