ABSTRACT
At zero frequency (dc) and at low frequencies, the transistor or FET amplifier stage requires an input voltage E1 equal to the sum of the input voltages of the device (the transistor Vbe or FET Vgs) and the voltage across the resistance Re or Rs between the common node (ground) and the emitter or source terminal. The input current I1 to the amplifier stage is equal to the sum of the current through the external resistor connected between ground and the base or gate and the base current Ib or gate current Ig drawn by the device. In most FET circuits, the gate current may be so small that it can be neglected, while in transistor circuits the base current Ib is equal to the collector current Ic divided by the current gain beta of the transistor. The input resistance R1 to the amplifier stage is equal to the ratio of input voltage E1 to input current I1.
