Patent GB-995879-A (GB)
Title: A signal amplifier
IPC: H03F3/30
Filed: 1962-06-06
Abstract: 995,879. Transistor amplifying circuits. RADIO CORPORATION OF AMERICA. June 6, 1962 [June 19, 1961], No. 21940/62. Heading H3T. A power amplifier stage comprises at least two series-connected transistors and an output load and a power supply connected in series therewith, input to the amplifier stage being applied between emitter and base of a first one of said transistors whose collector is connected to the emitter of a second one of said transistors so that the first transistor functions as a common-emitter amplifier and the second transistor functions as a common-base amplifier, there being further provided a potentialdivider chain connected in series with the load and source, tapping points thereon being connected to the base electrodes of the transistors, and a capacitor connected across a resistor of the potential-divider chain so arranged that a large amplitude output across the load causes the base of the second transistor to become forward biased with respect to its collector whereby the transistor saturates, the first transistor however not saturating until an input signal of limiting magnitude is applied thereto, whereafter the circuit ceases to amplify linearly. Fig. 1 shows an amplifier comprising two such transistor pairs 8, 9 and 10, 11 connected as a single-ended push-pull stage having output load 138 connected between their common point and earth and preceded by input common-emitter stage 5, commoncollector stage 6 and driver common-emitter stage 7 coupled to the push-pull stage through transformer 65. Considering one side only of the output stage, bias for transistors 8 and 9 is derived from a potentiometer chain 118, 116, 101 and 100, the static bias for transistor 9 being developed across forward-biased diode 100. A large-valued (500 microfarad) capacitor 130 is connected across resistor 101 and diode 100 so that charges across load 138 appear also at point 115, the static bias at 115 being such that for large amplitude signals the point 115 goes negative with respect to line 15 sufficiently to permit the collector-base diode of the transistor to become forwardly biased and the transistor to saturate. The circuit, however, continues to amplify linearly until the input amplitude becomes large enough to cause transistor 9 to saturate. The other side of the output stage, transistors 10 and 11, function similarly. In the preceding stages, which are energized from line 17, in turn supplied from line 15 through voltage-dropping circuit 20, negative feedback is supplied to the base of transistor 5 from the emitter of transistor 6. Emitter bias for transistor 5 is derived from silicon diode 36. Transistors 6 and 7 have common emitter resistors connected through resistor 43. Negative feedback from the output to transistor 7 is applied via these emitter resistors, resistor 52 providing voltage feedback and resistor 53 current feedback. Distortion due to directcurrent flow in the primary winding of transformer 65 is reduced by coupling the transformer for A.C. to the transistor through capacitor 48 and for D.C. thereto through resistors 47 and 46. The D.C. feedback to the base of transistor 7 via the primary winding provides further stability. Fig. 2 (not shown) shows a modification in which a further transistor is connected in series with each of transistors 8 and 10.