Design Con 2015

Stable pulse generator uses matched transistors in a current mirror

Bill Morong, Paoli, PA; Edited by Margery Conner and Fran Granville -February 02, 2012

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Using CMOS gates to generate pulses sometimes causes timing uncertainty due to gate-threshold variations. For accurate pulse widths, you can use BJTs (bipolar-junction transistors). Basing the design on current comparison allows the circuits to operate at low voltages. Proper clamping of the timing capacitor avoids pulse shortening with increased repetition frequency. These circuits work with somewhat less accuracy at supply voltages lower than 5V.

The heart of this design is a current mirror using modern dual transistors. Process improvements have made many ordinary dual transistors inherently well-matched. Testing a statistically significant sample of PMBT856 devices typically yields a better-than-1-mV match and no mismatches at voltages greater than 2 mV. As has been true for decades, PNP-transistor pairs are better matched than NPN transistors. Testing PMBT3904 devices yields 2-mV matches, with none worse than 3 mV. The packages measure approximately 2 mm on a side, which gives good thermal coupling between the pair. A current mirror with devices having 2-mV mismatch has 8% error. Devices with 3-mV mismatch yield 12% current error. Even with these errors, the circuit makes pulses that are more predictable than those that CMOS devices make.

Stable pulse generator uses matched transistors in a current mirror figure 1Figure 1 depicts a simple implementation of a current-mirror pulse generator. It provides good performance over a 0 to 100°C temperature range (Figure 2). Stable pulse generator uses matched transistors in a current mirror figure 2The closely spaced traces in the waveforms of these circuits are the 0 and 100°C outputs. Source V2 produces a 40-kHz square wave with a 33% duty cycle. The negative transition of this wave produces a peak current of 4 mA in timing capacitor C1. A time constant of 4.7 μsec is set with the value of resistor R1. The timing current of C1 and R1 passes through diode-connected transistor Q1, which, being connected in parallel with the base-emitter junction of Q2, forms a current mirror that replicates in Q2 the timing current in C1 and R1. Because the base-to-emitter-voltage-to-emitter-current curves of Q1 and Q2 match and Q1 and Q2 are at the same temperature, Q2 current matches Q1 current. A quiescent current of about 0.85 mA is set in R3. When the timing pulse increases Q2’s current to exceed R3’s quiescent current, Q3 lacks base current and turns off, initiating a negative pulse across load resistor R4.

Read more design ideasWhen the timing current decays below the quiescent current of R3, base current flows into Q3, turning it on and terminating the pulse on R4. Q2 saturates early in this pulse and becomes less saturated as the timing current decays.

When V2 transitions positive, it drives the bulk of its current into D1, yielding a short recovery time constant. D1 ceases to conduct at one diode drop above V1’s supply voltage, so the recovery tail from that diode drop to the quiescent base voltage of Q1 depends on the current decay in R1, which is a longer time constant. This simple circuit is stable, varying less than 4% over 100°C.

Stable pulse generator uses matched transistors in a current mirror figure 3Although stable, this circuit does not provide high-speed operation. In the circuit’s quiescent state, there is no current in either Q1 or Q2, making for a low gain bandwidth. Also, Q3 is in saturation, delaying the initial fall of the pulse across R4 because the free carriers must leave the base region. Q2 also saturates during the pulse, delaying the rise at the end of the pulse.

Stable pulse generator uses matched transistors in a current mirror figure 4Figure 3 depicts an improved current-mirror pulse generator. In this circuit, the operation of C1, R1, and D1 follows that of Figure 1. Changing D1 to a Schottky diode reduces the recovery-tail voltage that R1 must dissipate. Add R2 to draw a keep-alive current of 100 μA through Q1 and Q2, speeding turn-on. These keep-alive currents need not affect the timing. You can cancel out their effect with a slight reduction in the value of R3. Fitting Q2 and Q4 with Schottky clamps D2 and D3, respectively, keeps the transistors out of saturation. These changes improve high-speed performance (Figure 4).

Stable pulse generator uses matched transistors in a current mirror figure 5Although improved, the circuit still relies on D1 for the final tail of recovery. To eliminate this problem, you can replace D1 with a fourth transistor, Q4 (Figure 5). Because transistors Q1 and Q2 are slightly conducting, a voltage one diode drop below that of supply V1 is always present at their bases. You filter this voltage with R5 and C2 and provide it as a bias to the base of Q4. This step keeps Q4 nearer the threshold of conduction than would a diode to supply V1. When source V2 changes to a negative state, Q4 is fully off and draws no current. When V2 changes to a positive state, the emitter of Q4 conducts at voltages above V1 to catch the recovery transition, Stable pulse generator uses matched transistors in a current mirror figure 6further reducing the recovery-tail amplitude.

R6 may be used to limit Q4’s base current, but its omission is acceptable if source V2 has sufficient output resistance. It may be destructive to apply source V2 swings large enough to cause excess reverse voltage across the Q4 base-emitter junction. Q3 and Q4 can share the same package. These additions further improve the pulse generator’s high-speed performance (Figure 6).

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