Patent GB-739829-A (GB)
Title: Improvements in or relating to semi-conductor amplifiers
Inventor: CHAPMAN ROGER
IPC: H03F1/34
Filed: 1954-02-10
Abstract: 739,829. Deaf aids. KOLSTER-BRANDES, Ltd. Feb. 10, 1954, No. 3908/54. Class 40 (4). [Also in Group XL (c)] In a cascaded transistor amplifier with D.C. connections between stages, D.C. negative feed-back is arranged between a later and an earlier stage to stabilize the collector current of the later stage. The emitters are connected to a fixed reference point for A.C. in order to obtain maximum power gain and the collector of each transistor is connected to the base of the next. The circuits are particularly applicable to hearing aids, and additional negative feed-back may be applied to stabilize the gain. Input signals are fed over transformer 14, Fig. 1, to the base-emitter circuit of PNP transistor 1 having the emitter 5 biased appropriately by potential divider 11, 13. The emitter and lower end of transformer 14 are held at the potential of line 10 for signal frequencies by by-pass capacitors 15, 20. The high impedance circuit of collector 7 is coupled to the low impedance base-emitter circuit of transistor 2 over the auto-transformer 16. The bias potentials of the emitter 6, base 4 and collector 7 are determined by resistors 12, 18, 17 by-passed by capacitors 21, 23 and 22 at signal frequencies, the bias on collector 8 being fixed by the resistance of the output load 19. D.C. negative feed-back is obtained by a connection from the emitter 6 over resistor 24 and transformer 14 to the base 3. If the collector current of transistor 2 increases, the emitter current also increases and the potential falls relative to line 10, the base 3 also being carried negative so that the current flowing through collector 7 increases and its potential rises. The rise is transmitted to the base 4 thereby tending to reduce the emitter-collector current and thus stabilizing it. The collector current of transistor 1 is also stabilized. The windings of the transformer 16 are in opposition to give a step-down effect, or may be in the same direction in which case the resistor 18 is connected to a tapping on the single winding. By a suitable choice of resistor 12 and the resistance of transformer 16 many of the components may be omitted so that a four-stage amplifier appears as in Fig. 3. The resistance of the windings of transformer 16 may be so chosen that resistor 18 may also be omitted. When an odd number of stages is used, Fig. 4 (not shown), the feed-back is from the emitter of the final transistor 2 to the emitter of the first transistor 1, but due to the negative biasing of the emitter relative to line 10 the base is made more negative by the addition of a potential divider. In a modification, Fig. 5, the feed-back is taken from the collector of transistor 2 over resistor 28 to the base of transistor 1, the emitter being biased negatively with respect to line 10 by divider 25, 26 to maintain a suitable emitter-base potential. Any of the transistors may be replaced by a NPN-type provided the connections to the source 9 are inverted. In the two-stage amplifier, Fig. 6, the emitter 31 of the NPN transistor is biased appropriately by resistor 33 by-passed by capacitor 34, the collector 32 being connected to the positive pole of the battery 9 over the coupling transformer 16. The base 30 is biased by resistors 25, 26 and the feed-back is applied over resistor 28. Hearing aid, Fig. 7. The current from an electromagnetic microphone 35 is tapped off potentiometer 39 and applied to the baseemitter circuit 40, 37 of the input transistor, the collector 36 being connected to input transformer 14 of a two-stage version of the amplifier of Fig. 2 (not shown). The overall gain of the amplifier is stabilized by a further negative feed-back path comprising resistors 38, 41, the base potential being determined by connection to resistor 12. Specifications 694,021 and 700,231, [Group XL (c)], are referred to.