Lecture Notes for Analog Electronics - The Electronic Universe ...

I P
R P
• Early Effect. VBE depends on VCE:
V cc
I L
R L
Figure 28: Current mirror.
∆VBE
∆VCE
≈−1×10 −4
• Miller effect. This affects high-frequency response. The reverse-biased “diode” between
base and collector produces a capacitive coupling. Just as emitter resistance
is effectively multiplied by β + 1 for input signals, so too this CCB, which is ususally
a few pF, appears to input signals as a capacitance (1 + G)CCB to ground, where G
is the voltage gain of the transistor configuration. Hence, when combined with input
source resistance, this is effectively a low-pass RC filter, and the amplifier response for
frequencies above the RC cutoff will be greatly reduced. The usual solution for mitigating
the Miller effect is to reduce the source impedance. This can be effectively done
by coupling to a second transistor with small source resistance at base. The cascode
configuration, discussed in the text, uses this. Another example is the single-input
DC differential amplifier, for which there is no collector resistor at the input transistor
(this eliminates ∆VC even though the source resistance may be non-negligible), and
the output transistor has grounded base (therefore with very small source resistance),
• Variation in gain. The β may be quite different from transistor to transistor, even of
the same model. Therefore circuit designs should not rely on a specific gain, other
than to assume that β ≫ 1.
To illustrate this last point, consider our earlier one transistor current source. We determined
that the load current can be written
� ��VB �
β −VBE
IL =
(33)
β +1
RE + re
where the intrinsic emitter resistance re has been included. Therefore, the variation in IL
induced by variation in β is
∆IL
IL
= 1
IL
dIL
dβ
∆β =
33
�
1
�
∆β
β +1 β