Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER If the cathode current is still 2 mA, the voltage drop across Rj^ will be 20 V. If the negative supply is made —18 V the cathode will still be 2 V positive with respect to the grid and the bias is still negative even though it might appear from Figure that it is positive. The gain without feedback is now 50,000+10,000 _ ^. 10,000 A = 50 50,000 + 10,000 + 10,000 = 43 Bis 10,000 + 50,000 0-167 and AB = 7-2.
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER If the cathode current is still 2 mA, the voltage drop across Rj^ will be 20 V. If the negative supply is made —18 V the cathode will still be 2 V positive with respect to the grid and the bias is still negative even though it might appear from Figure that it is positive. The gain without feedback is now 50,000+10,000 _ ^. 10,000 A = 50 50,000 + 10,000 + 10,000 = 43 Bis 10,000 + 50,000 0-167 and AB = 7-2. If it be allowed that 7-2 is ^1, then it may be stated straight off that the gain is approximately 1/5 = 6. The exact gain expression is Al(\ + AB) = 43/(1 + 7-2) = 5-25. If f^ falls to half the new gain is 21-5/(1 + 3-6) = 4-7. As a result of increasing Rj^ the change in gain with feedback for a halving of valve [jl falls from 35 to 10-5 per cent—a desirable improvement. CONCERTINA PHASE SPLITTER When R^ = R^^, dVji^ and dVn^ are equal and opposite, and the device is called a phase splitter (Figure ). When the grid is made positive, for HT* example, the valve current increases and so do the drops across R^ and i?^; therefore the anode goes negative and the cathode positive. As the grid potential alternates the anode and grid may be visualized as going 'in and out' concertina-wise. The gain with feedback is dV in (5Fi an and is 1 + AB where B and A julRj^ ra + 2Rl It is about 2. The gain, taking the output between either output terminal and earth is {^^Rj)l{^^in) and is therefore about unity. 168
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