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Figure 2: Adding an emitter resistor decreases gain, but increases linearity and stability. Common-emitter amplifiers give the amplifier an inverted output and can have a very high gain that may vary widely from one transistor to the next. The gain is a strong function of both temperature and bias current, and so the actual gain is somewhat ...
Diagram from Widlar's original patent. 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.
See also an example of a mirror with emitter degeneration to increase mirror resistance. For the simple mirror shown in the diagram, typical values of will yield a current match of 1% or better. Figure 2: An n-channel MOSFET current mirror with a resistor to set the reference current I REF; V DD is positive voltage.
Resistor, R1, supplies the Zener current and the base current (I B) of NPN transistor (Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2. Voltage across R 2 (V R2) is given by V Z − V BE, where V BE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by
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 ...
A lower base-resistor voltage drop reduces the base current, which results in less collector current because I c = β I b. Collector current and emitter current are related by I c = α I e with α ≈ 1, so the increase in emitter current with temperature is opposed, and the operating point is kept stable.
The Wilson current mirror achieves the high output impedance of equation (6) by negative feedback rather than by emitter degeneration as cascoded mirrors or sources with resistor degeneration do. The node impedance of the only internal node of the mirror, the node at the emitter of Q 3 and the collector of Q 2, is quite low. [3]
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]