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Transistor with a Controlled Beta

IP.com Disclosure Number: IPCOM000093253D
Original Publication Date: 1967-Jul-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Berding, AR: AUTHOR

Abstract

Current gain can be accurately predicted for the transistor pair connected in the left-hand circuit. The circuit utilizes the principle that the voltage drop across a base-emitter junction is largely a function of the current density. Total current through the transistor is then the product of current density times the emitter area.

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Transistor with a Controlled Beta

Current gain can be accurately predicted for the transistor pair connected in the left-hand circuit. The circuit utilizes the principle that the voltage drop across a base-emitter junction is largely a function of the current density. Total current through the transistor is then the product of current density times the emitter area.

If a pair of transistors T1 and T2 is connected as shown, the current density at the base-emitter junction of T1 is the same as the current density at the base- emitter junction of T2. This is because the junctions are in parallel and any difference in current density is accompanied by a change in the voltage across the junction. Such action tends to equalize the current density.

Assuming that the emitter area in T1 is the same as that of T2, the circuit operates as a transistor with a current gain, or beta, of 1. Present technology allows the relative emitter areas of T1 and T2 to be accurately controlled. By suitable proportioning of emitter areas the desired beta can be obtained.

Another application of this principle occurs in the complementary class-B amplifier in the right-hand circuit. Transistor pairs T3, T4 and T6, T7 operate as emitter-followers with a negative Vbe. As the input signal at terminal 1 goes more positive, T3 tends to turn off. This diverts current through resistor 2 from T3 to the base of T4. The resulting increased base current decreases the impedance of T4 to raise the voltage...