8
Small signal analysis (cont.)
Av = gm 1 gm + RLtot
RLtot 1 1 = Rs g ds g m b
• Interesting cases
– Rs→∞, ro→∞, gmb=0
• PMOS, source tied to body, ideal current source
CMOS Chapter3
1 rin ≈ ( g m + g mb )
RD rin ≈ ( g m + g mb )ro
21
Small Signal Output Resistance
rout = [1 + ( g m + g mb )ro ]RS + ro
(Very high if (gm+gmb)RS>>1
Zou Zhige
CMOS Chapter3 18
Small signal analysis (2)---- CG Current Transfer
Define:
Zou Zhige
CMOS Chapter3
gm’ = gm + gmb
19
CG Current Transfer (cont.)
io RS ( g m + g mb ) = ii 1 + RS ( g m + g mb ) + RD / ro
Zou Zhige
CMOS Chapter3
∂VTH (注意： ≠ 0) ∂Vin
6
Small signal analysis of SF From SSQC
Zou Zhige
CMOS Chapter3
7
Small signal analysis (cont.)
Zou Zhige
CMOS Chapter3
Zou Zhige
CMOS Chapter3
14
Table of contents • Introduction • Common-Source Amplifiers • Source follower Amplifiers • Common Gate Amplifiers • Cascode Amplifiers
Reduced input and output voltage swing
– Consider e.g. VDD=1V, VTH=0.3V, VOV=0.2V ! CD buffer stage consumes 50% of supply headroom! – In low VDD applications that require large output swing, using a CD buffer is often not possible – CD buffers are more frequently used when the required swing is small ! E.g. pre-amplifiers or LNAs that turn μV into mV at the output
Zou Zhige
CMOS Chapter3
15
Common Gate Stage
Fig.1 CG stage with (a) direct coupling at input (b) capacitive coupling at input.
•
In a common-gate amplifier, the input signal is applied to the source terminal, as is shown in Fig.1. It senses the input at the source and generate the output at the drain. The gate is connected to a dc voltage to establish proper operating conditions. Note that in Fig.1(b) the bias current of M1 flows through the input signal source.
Zou Zhige
CMOS Chapter3
11
Application 1: Level Shifter
• Output quiescent point is roughly VTH+Vov lower than input quiescent point
Why the VGS is constant?
•
After that, M1 is driven into the triode region.
CMOS Chapter3 17
Zou Zhige
Small signal analysis (1)---- Voltage Gain
vout ( g m + g mb )ro + 1 Av = = RD vin ro + ( g m + g mb )ro RS + RS + RD
Zou Zhige
CMOS Chapter3 3
Source Follower (cont.)
Input terminal: gate; output terminal: source.
Zou Zhige
CMOS Chapter3
4
Large signal behavior of SF
• • • When Vin<VTH, M1 is off, and Vout is 0. When Vin>VTH, M1 turns on in saturation. As Vin increases further, Vout follows Vin with a difference of VGS. When Vin increases to a certain voltage (exceeding VDD), M1 enters triode region, the output voltage flattens out and clips at VDD.
Zou Zhige
CMOS Chapter3
5
Small signal analysis of SF
• In saturation region:
Vout =
∂Vout ∂Vin
1 W μ n Cox (Vin − VTH − Vout ) 2 ⋅ Rs 2 L ∂V 1 W ∂V = μ n Cox 2(Vin − VTH − Vout )(1 − TH − out ) ⋅ Rs 2 L ∂Vin ∂Vin
• We have: • So:
∂Vout ∂Vin
Vout = VSB
∂VTH =η ∂VSB
W 2(Vin − VTH − Vout ) ⋅ Rs L = W 1 + μ nCox 2(Vin − VTH − Vout ) ⋅ Rs (1 + η ) L
μ nCox
g m Rs AV = 1 + ( g m + g mb ) Rs
CMOS Chapter3 16
Zou Zhige
Large signal behavior
• •
When Vin>VB-VTH, M1 is off, and Vout is VDD. As Vin decreases, so does Vout, M1 is in saturation until
Zou Zhige
CMOS Chapter3
2
3.3 Source Follower (Common Drain Amplifiers)
• Our analysis of the CS stage indicates that, to achieve a high voltage gain with limited supply voltage, the load impedance must be as large as possible. If such a stage is to drive a lowimpedance load, then a “buffer” must be placed after the amplifier so as to drive the load with negligible loss of the signal level. The source follower (also called the commondrain stage) can operate as a voltage buffer. • The common drain stage exhibits high input impedance and low output impedance. After an analysis of relevant port characteristics, we will discuss some potential applications and also drawbacks of this circuit.
Chapter 3 Single-Stage Amplifiers
Zou Zhige 2007 HUST
Zou Zhige
CMOS Chapter3
1
Table of contents • Introduction • Common-Source Amplifiers • Source follower Amplifiers • Common Gate Amplifiers • Cascode Amplifiers
9
Small Signal Output Resistance