| 摘要 |
1,210,366. Automatic gain control. SIE. MENS A.G. 7 July, 1969 [8 July, 1968], No. 34107/69. Heading H3T. A D.C. amplifier comprising transistors T 3 , T 4 , T 5 is provided with D.C. feedback over resistor R 4 and capacitor C 1 . In series with the load resistor R 2 of the first stage T 3 is the emitter to collector path of a further transistor T 2 ; this transistor is provided with a standing bias by means of resistor R 1 , and the gain of the amplifier is varied by variation of its base bias. This varies the current through transistor T 2 , tending to change the potential at the point b; as a result the D.C. negative feedback on the circuit changes the bias on transistor T 3 arid hence the current through it in such a manner as to restore the potential of point b to its original value. In this way the current through transistor T 3 and hence its gain is varied without appreciable variation in the working conditions of the transistors T 4 , T 5 . The bias on the transistor T 2 may be varied by means of a further transistor (T 1 , Fig. 3, not shown) having its emitter connected to earth and its collector connected to the base of transistor T 2 via a diode chain. Variation of the base bias of this further transistor controls the current in the transistor T 2 , the diodes setting a minimum value beyond which the gain of the amplifier remains constant; the variation of collector potential may be used to vary the gain of preceding stages, e.g. R.F. stages of a radio receiver in which the amplifier described constitutes the I.F. stage. |
