tailieunhanh - analog bicmos design practices and pitfalls phần 8

Nhắc lại rằng gm = Ic / VT và Ro = 1/gm + Rs / β, chúng tôi lưu ý rằng cả hai trong số này số lượng phụ thuộc vào các nhà sưu tập hiện tại của Q1. Chúng tôi đã lưu ý rằng biến động rộng rãi trong các nhà sưu tập hiện tại có thể được sử dụng như là một giai đoạn đầu ra, | Ro R 9m p Recalling that gm Ic Vt and Ro 1 gm Rs p we note that both of these quantities are dependent on the collector current of Q1. We have already noted that wide swings in collector current can be expected in use as an output stage and so the use of small signal analysis for the emitter follower output stage must be carefully considered. Reasonable estimates of both voltage gain and output resistance can be obtained by using quiescent bias current in the equations as long as the input voltage changes are moderate. The Common-Emitter Class A Output Stage The common-emitter circuit is another frequently used output stage. It has advantages over the emitter follower stage in that voltage gain is possible and the output voltage can swing closer to the supply rails. However base-collector capacitance introduces phase shifting and results in the need for frequency compensation while operating the output near the supply rails results in saturated output transistors and introduces signal distortion. An example common-emitter output stage is shown in Figure . We again begin our analysis by noting that the supply voltages are of equal magnitude and opposite polarity that the load is ground-referenced and that a transistor current source comprised of Q2 RE and a bias voltage are present. The input voltage is referenced to VCC to simplify the analysis. Current source Q2 provides a quiescent current denoted Iq. The output current is then given as Io Iq Ic Q1 We also have Vo IoRl If we combine these two equations and use the diode equation to express Ic Q1 as a function of VBE1 we obtain Vo RlVEXp V Iq The transfer characteristic is shown in Figure . While there are similarities to the transfer characteristic of the emitter follower circuit there are two very important differences. The first is that the transfer characteristic for the common-emitter stage is exponential. This means that as Vị changes the output signal Vo will exhibit Figure Common-emitter .