GB1233353A - - Google Patents

Abstract

1,233,353. Transistor circuits for insulating a diode. INTERNATIONAL BUSINESS MACHINES CORP. 6 Feb., 1969 [22 March, 1968], No. 6446/69. Heading H3T. A circuit for simulating a diode characteristic has a diode D and resistor R5 connected from an input terminal 1 to one pole of a voltage source 3, a control circuit having an emitter coupled pair T1, T2 with the base of transistor T1 connected to the input terminal and the base of transistor T2 connected to a bias potential, and biasing transistors T3, T4 controlled by the control circuit T1 T2 to bias the diode conductive whenever the potential on the input terminal 1 exceeds the bias potential. As shown, the circuit uses NPN transistors and simulates a diode having the anode as 1 and the cathode as 2. When the input 1 is negative transistor T1 and diode D are off and transistors T3 and T4 are on. As the input increases positively transistor T1 conducts. This decreases the emitter potential of T4 and when the emitter potential of T4 becomes more negative, e.g. by 0.5V than the input 1 diode D conducts and current flows through R5 to the cathode 2. In a modification (Fig. 3, not shown), the bias potential for T2 is obtained from a tapping on a voltage divider (P) connected across the voltage source. Also a further voltage source (4) may be connected between R5 and via an external circuit (5) to the input terminal 1. Alternatively one of the voltage sources may be formed by tapping the other. The circuit may be made in monolithic semiconductor wafers and uses include limiters and amplifier input stages. Specification 1,215,334 is referred to.