Baker clamp

Baker clamp is a generic name for a class of electronic circuits that reduce the storage time of a switching bipolar junction transistor (BJT) by applying a nonlinear negative feedback through various kinds of diodes. The reason for slow turn-off times of saturated BJTs is the stored charge in the base. It must be removed before the transistor will turn off since the storage time is a limiting factor of using bipolar transistors and IGBTs in fast switching applications. The diode-based Baker clamps prevent the transistor from saturating and thereby accumulating a lot of stored charge.

The Baker clamp is named for Richard H. Baker, who described it in his 1956 technical report "Maximum Efficiency Transistor Switching Circuits." Baker called the technique "back clamping", but the circuit is now called a Baker clamp. Many sources credit Baker's report for the two-diode clamp circuit. Also in 1956, Baker described the circuit in a patent application; the 1961 issued patent, US 3,010,031, claims the use of the clamp in symmetrical flip-flop circuits.

Similar clamp circuits are said to have been known before Baker's report. Kyttälä states, "Although invention of the Baker Clamp circuit is credited to Richard H. Baker (US Patent 3,010,031) it was already common knowledge in 1953 and described in transistor introductory papers that were written by Richard F. Shea." However, Shea's 1953 transistor text does not describe a similar clamp circuit. Shea's 1957 text does describe the clamp circuit and references Baker's technical report.

There are other clamp circuits. A 1959 manual describes a technique called "saturation clamping". In that scheme, there is a saturation clamp supply at about 2 volts connected to the collector with a saturation clamp diode. When the transistor nears saturation, the clamp diode turns on and supplies the extra collector current to keep the transistor from saturating. The saturation clamp supply needs to supply substantial current. In contrast, the Baker clamp reduces the transistor base current rather than supplying more collector current.

Another clamp circuit uses a single diode clamp. It reduces base drive as the transistor nears saturation, but it uses a resistor divider network.

Clamp circuits were also used to speed up cutoff transitions. When the transistor is cutoff, the output is similar to an RC circuit that exponentially decays to its final value. As the circuit gets closer to its final value, there is less current available to charge the capacitor, so the rate of approach lessens. To reach 90 percent of the final value takes about 2.3 time constants. Cutoff clamping reduces the output voltage swing but makes the transition faster. Clamping the collector voltage to 63 percent of the final value allows a factor of two speed increase.

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