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Current gain in the common emitter circuit is obtained from the base and the collector circuit currents. Because a very small change in base current produces a large change in collector current, the current gain (β) is always greater than unity for the common-emitter circuit, a typical value is about 50.
A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values. [1] [2] [3]
Any change in output current with output voltage results in a change in the emitter current of Q 3 but very little change in the emitter node voltage. The change in is fed back through Q 2 and Q 1 to the input node where it changes the base current of Q 3 in a way that reduces the net change in output current, thus closing the feedback loop.
Imagine in Figure 5, at power up, that the LED has 1 V across it driving the base of the transistor. At room temperature there is about 0.6 V drop across the V be junction and hence 0.4 V across the emitter resistor, giving an approximate collector (load) current of 0.4/R e amps. Now imagine that the power dissipation in the transistor causes ...
Full hybrid-pi model. The full model introduces the virtual terminal, B′, so that the base spreading resistance, r bb, (the bulk resistance between the base contact and the active region of the base under the emitter) and r b′e (representing the base current required to make up for recombination of minority carriers in the base region) can be represented separately.
If T1 and T2 have the same base-emitter junction areas and operate at the same junction temperature, then in absence of input voltage the current flowing through T1 is mirrored in the output stage. The current replication ratio may be altered by scaling the transistors, or by the insertion of degeneration resistors in series with the emitters ...
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A load line diagram, illustrating an operating point in the transistor's active region.. Biasing is the setting of the DC operating point of an electronic component. For bipolar junction transistors (BJTs), the operating point is defined as the steady-state DC collector-emitter voltage and the collector current with no input signal applied.