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Open emitter output exposes the emitter as the output. [2] For an NPN open emitter output, the collector is connected to the positive voltage rail, so the emitter outputs a high voltage when the transistor is on and is hi-Z when off. For a PNP open emitter output, the collector is connected to the low voltage supply, so the emitter outputs a ...
The transistor continuously monitors V diff and adjusts its emitter voltage to equal V in minus the mostly constant V BE (approximately one diode forward voltage drop) by passing the collector current through the emitter resistor R E. As a result, the output voltage follows the input voltage variations from V BE up to V +; hence the name ...
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.
This causes a base current and much larger collector current to flow. The positive half-cycle of the signal is amplified in the collector. During the negative half-cycle, the base-emitter junction is reverse biased and hence no current flows. No output flows during the negative half-cycle of the signal.
The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is the usual exponential current–voltage curve of a p–n junction (diode). [3]
Due to transistor action, emitter current, I E, is very nearly equal to the collector current, I C, of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect ) and the circuit operates as a constant current source.
where V T is the thermal voltage, and I E is the DC emitter current. For example, for V T = 26 mV and I E = 10 mA, rather typical values, R in = 2.6 Ω. If I E is reduced to increase R in , there are other consequences like lower transconductance, higher output resistance and lower β that also must be considered.
In electronics, emitter-coupled logic (ECL) is a high-speed integrated circuit bipolar transistor logic family. ECL uses an overdriven bipolar junction transistor (BJT) differential amplifier with single-ended input and limited emitter current to avoid the saturated (fully on) region of operation and the resulting slow turn-off behavior. [2]