![]() Ī corresponding NMOS transistor configuration with source degeneration ( Rs ) is shown in Figure 4. Therefore, the feedback circuit implementation is simply a resistor divider with resistances ( R C – R) and R respectively. Forward Block A in Figure 3 can be implemented as the original transistor circuit with degeneration resistor ac shorted (using a bypass capacitor, for example).By noting that V 2 /V 1 = -A/(1+AB)= (- A)/(1-(-A)B), an alternate implementation of the negative feedback model is shown in Figure 3, with A = R C g m and with B = α -1 R e /R C.Again this is consistent with the observation that emitter degeneration linearizes a transistor amplifier output. If R e is such that α -1 g m R e > 1then the system gain is ≈ -α( R C /R e ), a constant, which implies output is a linear function of the input. If AB > 1, then the gain of the system ≈ -(1/B).This result is consistent with the observation made on transistor amplifiers with emitter degeneration. For larger values of R e, the negative feedback (B) increases and thus tends to make system gain more stable with respect to parameter variations in block A.The result is consistent with the common emitter circuit without a degeneration, namely, it is a forward circuit with a gain of – R C g m If R e = 0, then there is no feedback (B = 0) and the above model is a pure forward system.It is obvious that for small signal analysis, the emitter degeneration transistor can be modelled as a negative feedback system shown above with A = R C g m and with B = α -1 R e /R C. Notice the similarity between this equation and the small signal voltage gain of the emitter degeneration transistor, Δ V c/ Δ V bx. The gain of the system is V2/V1 = -A/(1+AB). In these situations, the emitter degeneration resistor comes to the rescue.įigure 2 A diagram of the small signal gain of Figure 1 transistor configuration with emitter degenerationįigure 2 shows a negative feedback system with feedforward gain of A and a feedback factor of B. In such cases the nonlinear behavior is readily apparent with a distorted output. However, many input signals to amplifiers are in the order of a few millivolts to a few hundred millivolts where approximation x → 0 is not applicable. ![]() Similarly, in a BJT differential amplifier, the transfer characteristic being a hyperbolic tangent function (tanh) is nonlinear.īecause of the properties of the exponential function ( e kx – 1 ≈ kxas x → 0) and that of the tanh function ( tanh(kx) ≈ kx as as x → 0)the nonlinear behavior of both the CE amplifier as well as the differential amplifier is well masked for signals whose input amplitudes are very small. The transfer characteristic of a BJT common emitter (CE) amplifier is exponential, making the output a highly nonlinear function of the input. In addition, it helps linearize the small signal output. Emitter/source degeneration is a technique used to guard against drift in transistor parameters like beta.
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