Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER output voltage d Vji^ + d Vr^. Therefore the input to the valve is the difference between the signal dVin and a fixed fraction of the output, so this is a negative feedback system, the feedback being applied round one stage only. The A A^ A2 ^3 1 5i Bi 03 Figure is {^i{RI^ + RK))l{ra + {Rl + Rk)) and the B is RkI{Rl + Rr)- Typically Rl might be 50,000 Q, /•„ = 10,000 Q, the bias 2 volts, the anode current
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER output voltage d Vji^ + d Vr^. Therefore the input to the valve is the difference between the signal dVin and a fixed fraction of the output, so this is a negative feedback system, the feedback being applied round one stage only. The A A^ A2 ^3 1 5i Bi 03 Figure is {^i{RI^ + RK))l{ra + {Rl + Rk)) and the B is RkI{Rl + Rr)- Typically Rl might be 50,000 Q, /•„ = 10,000 Q, the bias 2 volts, the anode current 2 milliamps and ^ equal to 50. From these figures R^ must be 1,000^, so the gain without feedback will be Our B 50(50,000 + 1,000) (10,000 + 50,000+ 1,000) 1,000 42 50,000+ 1,000 0-02 AB is therefore only 0-84, which is not ^1, and therefore the amount of feedback is small. If in course of time fj, falls to half its original value, the gain with feedback falls from 42/(1 + 0-84) to 21/(1 + 0-42), from 23 to 15.* The gain can hardly be said to be stabilized. To improve matters we need more Rj^. If we merely increase R^ in Figure the valve tries to give HT +
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