GB973066A - A transistor amplifier - Google Patents

Abstract

973,066. Transistors amplifying; switching circuits. LEEDS & NORTHRUP CO. April 3, 1963, No. 13221/63. Heading H3T. The collector load resistance of a common emitter input transistor is connected in a loop circuit including the emitter-collector path of a further transistor, and sources of collector potential and forward base-biasing potentials for the further transistor, a ground connection being made to the base-biasing potential such that the potential between ground and the base of the further transistor is effective through the conductive base-emitter junction of the further transistor to maintain the collector of the input transistor at a high potential and to maintain a low impedance between the collector and ground. The collector of input transistor 10 is connected as shown through common base transistor 11 to output terminal 13, transistor 11 being biased by sources 19, 21 to be conducting for all values of collector current of transistor 10 and maintained in a non-saturated condition by supply 24 and resistor 25. Since the emitter-base junction of transistor 11 is forward biased and hence has a low impedance, the collector of transitor 10 is kept at the constant negative potential of source 21. The effective capacitance between the collector and base of transistor 10 and hence the internal high-frequency feedback of this transistor is minimised due to the high constant potential difference between the collector and base. The feedback is further minimised by providing a low impedance path to ground for high frequencies since the point 31 is connected to ground through the low impedance path of transistor 11 and source 21. The emitter current of transistor 10 is limited to a maximum valve determined by the current from a constant current source 14, 15 which maximum valve is insufficient to saturate the transistor. If the input signal from 12b is such as to produce conductivity of transistor 10 below the maximum value the difference between the emitter current and the current from the constant current source flows through diode 16 and the amplifier acts as a common-emitter stage with high gain. As the input signal becomes more negative the emitter current increases until it has reached the value of the current from the source 14, 15 when diode 16 is reverse biased and the amplifier operates in a manner similar to an emitter follower. Limiting of the input signals is also achieved by including diode 30 in the circuit. Transistor 11 may be of PNP conductivity type or both transistors may be of NPN conductivity types.