Miller Sweep Generator

The Miller Sweep circuit is composed of Q630A, Q630B, Q631, and associated timing components. The circuit operates to hold the charging current to the timing capacitor at a constant value. When a capacitor is charged in this manner, the rise of voltage across the capacitor is linear rather than exponential.

Field-effect transistors Q630A and Q630B are matched devices. As such, the lDSS (drain current with gate-to-

source shorted) characteristics of each are nearly identical. FET Q630B acts as a source-current supply for Q630A and holds the gate-to-source voltage of Q630B at zero volts.

Before a sweep starts, pin 6 of U620 (the Sweep Logic Gate) is HI, and both disconnect diodes (CR626 and CR630) are forward biased. The charge on the selected timing capacitor will be zero volts. When U620 pin 6 goes LO, the disconnect diodes become reverse biased and the timing capacitor begins charging through the timing resistor to start the sweep.

The overall gain of the amplifier composed of source-follower Q630A and common-emitter amplifier Q631 is very high. As the timing capacitor charges, Q631 supplies feedback to the gate of Q630A to hold the gate voltage nearly constant. Voltage across the timing resistor is therefore constant, and the charging current to the timing capacitor is constant. The resulting voltage waveform produced at the collector of Q631 is a linear ramp.

When the sweep waveform amplitude reaches about +13 V, the End-of-Sweep Comparator (Q640) is biased on and the Sweep Logic circuit resets. Pin 6 of U620 goes HI to forward bias disconnect diode CR626, and the current through the timing capacitor reverses direction. The sweep output waveform drops rapidly until disconnect diode CR630 also becomes forward biased. At this point, the Sweep Generator is ready to start another sweep.

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