. Electronic apparatus for biological research . Tâ^â OWT^-vW- 4^0^ HT+ Figure Figure way of achieving negative current feedback is to include some resistance in series with the valve cathode (Figure ). Then we have V11 Combining Zl + Rk + ra I = and V = Kin - iRi HV va. Zl + ^a + (,U + \)Rk Comparing this with the expression for a stage without feedback having the same load and a valve of internal resistance /â¢â we see that the effect of the feedback is to increase the internal resistance from râ to r^ + (^ + \)Ric. This is seen better if we call it ^a(l + ^^ Rl^ N /â â(! +
. Electronic apparatus for biological research . Tâ^â OWT^-vW- 4^0^ HT+ Figure Figure way of achieving negative current feedback is to include some resistance in series with the valve cathode (Figure ). Then we have V11 Combining Zl + Rk + ra I = and V = Kin - iRi HV va. Zl + ^a + (,U + \)Rk Comparing this with the expression for a stage without feedback having the same load and a valve of internal resistance /â¢â we see that the effect of the feedback is to increase the internal resistance from râ to r^ + (^ + \)Ric. This is seen better if we call it ^a(l + ^^ Rl^ N /â â(! + ) Typically gâj is 10 niA/V and r^ 1,000 ohms, giving an 11-fold increase in the internal resistance of our constant-current generator. It must not be forgotten that the improvement is not obtainable without a loss of gain. The input has to be increased by a factor ^L-r ^a-r J<K ^a approximately by the same factor as the pentode internal resistance is increased. FlybackâDuring the forward stroke energy is stored in the magnetic field linked with the deflector coil, and when the anode current is suddenly 250
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