178 LNA DesignAnother advantage of the cascode amplifier is that adding another transistor improves the isolation between the two ports (very little reverse gain in the am-plifier). The disadvantage is that the additional transistor adds additional poles to the system. This can become a problem for a large load resistance, leading to rapid high-frequency-gain roll-off (–12 dB/octave compared to the previous –6 dB/ octave) and excess phase lag, which can cause problems if feedback is used. As well, an additional bias voltage is required, and if this cascode bias node is not properly decoupled, instability can occur. A further problem is the reduced signal swing at a given supply voltage, since the supply must now be split between two transistors instead of just one as for the simple common-emitter amplifier.7.1.4 The Common-Collector/Drain Amplifier (Emitter/Source Follower)The common-collector amplifier is a very useful general-purpose amplifier. It has a voltage gain that is close to 1, but has a high input impedance and a low output impedance. Thus, it makes a very good buffer stage or output stage. Also, it can be used to do a dc level shift in a circuit.The common-collector amplifier and its small-signal model are shown in Figure 7.6. The resistor RE may represent an actual resistor or the output resistance of a current source. Note that the Miller effect is not a problem in this amplifier since the collector is grounded. Since Cm is typically much less than Cp, it can be left out of the analysis with little impact on the gain. The voltage gain of this amplifier is given by: 111/( )1/vvos zA sAs P+æö=ç÷èø+ (7.20)Figure 7.6 Common-collector amplifier and its small-signal model.