University:
Massachusetts Institute of Technology
Course:
6.012 | Microelectronic Devices and Circuits
Academic year:

2021
Views:

136

Pages:

4
Author:

Kaylynn Erickson

Recitation 24 BiCMOS Cascode Ampliﬁer 6.012 Spring 2009 Recitation 24: BiCMOS Cascode Ampliﬁer Yesterday, we talked about Multistage ampliﬁers. The simplest way to have multi-stage ampliﬁers is to directly connect the output of the previous stage to the next stage, as we talked about yesterday. We call this way “cascade”. Today we will talk about another topology - “cascode” (Before doing that, we can have a simple exercise) What is the following ampliﬁer and what is its function? Figure 1: This is a CD-CC Voltage Buﬀer. Since Rin of CD is ∞, this can couple very well with the voltage signal of the previous stage, even there is a high Rout at the previous stage And CC Rout = Rout . 1 Rs + . gm (BJT) typically > gm (MOS) having CC could reduce ﬁnal βo gm BiCMOS Cascode Ampliﬁer We would like to put these two together. (You will see advantages later: MOS has large Rin and BJT has good frequency response) 1 Recitation 24 BiCMOS Cascode Ampliﬁer But instead “cascading”, we do the following: Let us look at the small signal model (low frequency) Rout = γo [1 + gm (γπ ||Rs )] ||γoc Rs = γo1 γo1 (MOS) typical 100 kΩ 2 6.012 Spring 2009 Recitation 24 BiCMOS Cascode Ampliﬁer 6.012 Spring 2009 γπ ∼ 10 kΩ, γπ ||Rs = γπ Rout = γo [1 + gm γπ ] ||γoc = βo γo ||γoc βo Transconductance Gain Gm = iout vs = −iin = vs ∼1 γo1 1 ·(gm vgs1 ) γo1 + gm2 vs = gm It is the same as CS ampliﬁer itself! Why would we want to add the CB stage? Frequency Response 1. Frequency response of CS Rout = γo ||γoc ||RL w3dB = 1 ) + R Rs Cgs + Rs Cgd (1 + gm Rout out · Cgd Miller eﬀect: Cgd multiplied (ampliﬁed) by gain gm Rout 2. Frequency response of CB 3 Recitation 24 BiCMOS Cascode Ampliﬁer Rin = w3dB = 1 , gm 6.012 Spring 2009 Rout = γoc ||γo βo 1 1 C (Rs ||Rin )Cπ + (Rout ||RL ) · Cμ π + Cμ · RL gm 3. Now combining the two together Apply OCT technique (+ Miller Approximation) for frequency response: (a) Cgs1 : RTHCgs = Rs 1 (b) Cgd1 : voltage gain across Cgd1 is Vgs1 γo || gm1 −gm gm V π2 2 = − − 1 Vgs1 Vgs1 gm2 ∴ CM = Cgd1 (1 − AvCgd1 )Cgd1 RTHCM = Rs 1 1 gm2 gm 2 (d) Cμ : Rout || RL RL (c) Cπ2 : γo1 || ⎛ ∴ w3dB = 1 ⎞ ⎜ gm ⎟ ⎜ ⎟ Cπ Rs Cgs1 + Rs Cgd1 ⎜1 + 1 ⎟ + + Cμ RL gm2 ⎠ gm2 ⎝ ≤1 For BiCMOS cascode, Miller eﬀect is drastically reduced compared to CS case. gm1 . Cgd is multiplied by 1 + gm2 BJT (gm2 ) ≥ gm1 (MOS) gm 1+ 1 ≤ 2 gm 2 Therefore, BICMOS cascode ampliﬁer has overall voltage gain of C-S, but with frequency response comparable to CB Ampliﬁer. 4