Posted on Feb 5, 2014

Those variations, (in effect an alternating signal), will be passed via the coupling capacitor to the next stage - shown by the arrow to the right. We can replace one variable resistor with another. Consider R1 to now be replaced with any one of the devices shown in this following diagram. Each can act as a variable resistor: Other connections mus

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t be made to the other electrodes of each device to bias it for correct operation so that the input signal can vary the bias which alters the internal resistance of the device, which: in turn varies the voltage at point A, which is passed on through the coupling capacitor to the next stage for further processing. The two resistors connected to the base of this transistor with the resistor in the emitter, form the biasing arrangements. These components set the level of the current flowing through the primary of the transformer - the collector current. An input audio signal is fed via a coupling capacitor to the base of the transistor. This varies the base current which in turn varies the effective resistance of the transistor and hence the collector current. The circuit below is a typical transmitter radio frequency amplifier. Here the load comprises all the components connected to the collector of the transistor Q1 - C5, L2, etc. and the antenna or whatever is connected to the output. The RF choke RFC2 feeds 12v DC to the device and prevents the RF from getting into the power supply leads. The input signal provides self-bias to the transistor. The conditions for oscillation are that the level of signal fed back is at an adequate level, and that the signal is in the correct phase to sustain oscillation. This is positive feedback. The feedback is taken by a secondary coil and inserts a signal in the base lead, changing the...

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