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common source vs. common gate

common source vs. common gate# EE - 电子工程

c*l2008-03-28 07:03

1 楼

这俩区别怎么想？
当然一个interverting,一个non-inverting
两个的gain好像基本差不多，Rout基本差不多
输入Rin呢，可能non-interverting小点？？
frequency response大概non-inverting好点因为没gate电容？
thanks
怎么来比较common source/common source degeneration的优缺点？
thanks thanks

d*a2008-03-28 07:03

2 楼

两个的gain好像基本差不多? How?

【在 c*******l 的大作中提到】

: 这俩区别怎么想？
: 当然一个interverting,一个non-inverting
: 两个的gain好像基本差不多，Rout基本差不多
: 输入Rin呢，可能non-interverting小点？？
: frequency response大概non-inverting好点因为没gate电容？
: thanks
: 怎么来比较common source/common source degeneration的优缺点？
: thanks thanks

c*l2008-03-28 07:03

3 楼

好像不一样。
但是我在直观的想
common source说把source voltage固定，动gate voltage，电流改变，
然后改变Rd的压降，得到输出，
common gate说把gate voltage固定，动source voltage，电流改变，
改变Rd压降，得到输出。在saturation mode里，I和Vgs直接一对一的话
变Vg和变Vs效果好像在I上一样
不知道为什么不一样了
两个都是high gain...

【在 d*****a 的大作中提到】

: 两个的gain好像基本差不多? How?

c*l2008-03-28 07:03

4 楼

重新看了一下，如果不记body effect，是一样，卡卡
Av=gm*Rout, (+/-)

【在 c*******l 的大作中提到】

: 好像不一样。
: 但是我在直观的想
: common source说把source voltage固定，动gate voltage，电流改变，
: 然后改变Rd的压降，得到输出，
: common gate说把gate voltage固定，动source voltage，电流改变，
: 改变Rd压降，得到输出。在saturation mode里，I和Vgs直接一对一的话
: 变Vg和变Vs效果好像在I上一样
: 不知道为什么不一样了
: 两个都是high gain...

b*m2008-03-28 07:03

5 楼

Low freuqncy gain is comparable.
CS is much better for low noise, and higher gain at high frequencies,
especially for cases where source degeneration inductances exist.
CG is mostly used in cascode.

【在 c*******l 的大作中提到】

: 重新看了一下，如果不记body effect，是一样，卡卡
: Av=gm*Rout, (+/-)

n*y2008-03-28 07:03

6 楼

then why typical amplifier, especially last stage of amp, it is C-S,
not CG, could u explain? :-)

【在 c*******l 的大作中提到】

: 重新看了一下，如果不记body effect，是一样，卡卡
: Av=gm*Rout, (+/-)

n*y2008-03-28 07:03

7 楼

C-S C-G
voltage gain
Rin
Rout
max vin swing to keep device in sat.
easeness of DC biasing
frequency response
......

then why typical amplifier, especially last stage of amp, it is C-S,
not CG, could u explain? :-)

【在 n*****y 的大作中提到】

: then why typical amplifier, especially last stage of amp, it is C-S,
: not CG, could u explain? :-)

f*02008-03-28 07:03

8 楼

those are for current gain. they are essentially 0db buffers.

【在 n*****y 的大作中提到】

: then why typical amplifier, especially last stage of amp, it is C-S,
: not CG, could u explain? :-)

c*l2008-03-28 07:03

9 楼

可能是因为虽然gain差不多，Rout也差不多，但是Rin的话，还是CS最大，所以好吧？？
卡卡

【在 n*****y 的大作中提到】

: then why typical amplifier, especially last stage of amp, it is C-S,
: not CG, could u explain? :-)

c*l2008-03-28 07:03

10 楼

gain差不多
rin是CS大
Rout也差不多
max vin的话，CS方便，因为S接地，但是如果CG的话，swing上线离gate至少要Vt，所
以不好
ease的话，还是CS方便些。要不G如果接低电压，S就得swing到negative去了
frequency的话，CG好，因为cap小，但是output stage估计多的是low freq了，
不要这个benefit?
嘿嘿

【在 n*****y 的大作中提到】

: C-S C-G
: voltage gain
: Rin
: Rout
: max vin swing to keep device in sat.
: easeness of DC biasing
: frequency response
: ......
:
: then why typical amplifier, especially last stage of amp, it is C-S,

c*l2008-03-28 07:03

11 楼

恩，可以这么理解么？
low noise是因为input cap有滤波，而source inductance呢可以因为source相对
的low impedance(compare to gate)抵消一下？
thanks

【在 b*****m 的大作中提到】

: Low freuqncy gain is comparable.
: CS is much better for low noise, and higher gain at high frequencies,
: especially for cases where source degeneration inductances exist.
: CG is mostly used in cascode.

n*y2008-03-28 07:03

12 楼

forget one thing,
what is the headroom of vout for CS and CG (if voltage supply rail is 0V and
Vdd)?

【在 c*******l 的大作中提到】

: gain差不多
: rin是CS大
: Rout也差不多
: max vin的话，CS方便，因为S接地，但是如果CG的话，swing上线离gate至少要Vt，所
: 以不好
: ease的话，还是CS方便些。要不G如果接低电压，S就得swing到negative去了
: frequency的话，CG好，因为cap小，但是output stage估计多的是low freq了，
: 不要这个benefit?
: 嘿嘿

c*l2008-03-28 07:03

13 楼

CS的话应该是Isat*Rd吧， CG也是？？

and

【在 n*****y 的大作中提到】

: forget one thing,
: what is the headroom of vout for CS and CG (if voltage supply rail is 0V and
: Vdd)?

n*y2008-03-28 07:03

14 楼

Do u understand "headroom"?

【在 c*******l 的大作中提到】

: CS的话应该是Isat*Rd吧， CG也是？？
:
: and

b*m2008-03-28 07:03

15 楼

at low frequency, CS signal output resistance doesn't contribute the total
output noise too much, but in CG, its contribution is comparable to the
transistor noise source.

【在 c*******l 的大作中提到】

: 恩，可以这么理解么？
: low noise是因为input cap有滤波，而source inductance呢可以因为source相对
: 的low impedance(compare to gate)抵消一下？
: thanks

c*l2008-03-28 07:03

16 楼

en. 你指教？

【在 n*****y 的大作中提到】

: Do u understand "headroom"?

b*m2008-03-28 07:03

17 楼

they are the same, for small signal analysis

and

【在 n*****y 的大作中提到】

: forget one thing,
: what is the headroom of vout for CS and CG (if voltage supply rail is 0V and
: Vdd)?

c*l2008-03-28 07:03

18 楼

thanks thanks

【在 b*****m 的大作中提到】

: at low frequency, CS signal output resistance doesn't contribute the total
: output noise too much, but in CG, its contribution is comparable to the
: transistor noise source.

c*h2008-03-28 07:03

19 楼

my 2 cents
1. CG has low input impedance 1/gm, so it can be used as input stage in high
speed design for the impedance match 75 ohm. Unlike inductance RF design,
it offers wideband.
2. For output stage, follower is often used because we want low impedance to
drive high impedance for a voltage source. C-G is opposite.
3. For noise consideration, C-G and C-S have same noise performance.
C-S is Vsignal/Vnoise (both refered to gate)
C-G i signal = V signal / Rin (Rin = 1/gm)
i noise = v noise gm, so

【在 c*******l 的大作中提到】

: en. 你指教？

h*y2008-03-28 07:03

20 楼

for CS you need not consider the loading effect to previous stage, but
CG has such problem.

c*l2008-03-28 07:03

21 楼

nod. CG的Rin大概是小些

【在 h*******y 的大作中提到】

: for CS you need not consider the loading effect to previous stage, but
: CG has such problem.

le2008-03-28 07:03

22 楼

could you please explain 4 a little more?

high
to

【在 c******h 的大作中提到】

: my 2 cents
: 1. CG has low input impedance 1/gm, so it can be used as input stage in high
: speed design for the impedance match 75 ohm. Unlike inductance RF design,
: it offers wideband.
: 2. For output stage, follower is often used because we want low impedance to
: drive high impedance for a voltage source. C-G is opposite.
: 3. For noise consideration, C-G and C-S have same noise performance.
: C-S is Vsignal/Vnoise (both refered to gate)
: C-G i signal = V signal / Rin (Rin = 1/gm)
: i noise = v noise gm, so

C*g2008-03-28 07:03

23 楼

power gain差不多,但是voltage gain差了

【在 d*****a 的大作中提到】

: 两个的gain好像基本差不多? How?

C*g2008-03-28 07:03

24 楼

I think at high frequencies the noise and gain performance will be
comparable, though CS will be a bit better;
And source degeneration only REDUCES gain, right?

【在 b*****m 的大作中提到】

C*g2008-03-28 07:03

25 楼

because CS can provide both current and voltage gain. It will be helpful to
have max. voltage and current gain at the last stage of an opamp if the
amplifier is configured in feedback configuration.
Besides, for the max. driving capability, the last stage of an opamp is
sometimes class A/B, if not all the times. I saw a lot more CS configured
class-AB than CG, maybe because it is easier to implement the bias? My guess.

【在 n*****y 的大作中提到】

: then why typical amplifier, especially last stage of amp, it is C-S,
: not CG, could u explain? :-)

C*g2008-03-28 07:03

26 楼

and also the channel noise

【在 b*****m 的大作中提到】

: at low frequency, CS signal output resistance doesn't contribute the total
: output noise too much, but in CG, its contribution is comparable to the
: transistor noise source.

C*g2008-03-28 07:03

27 楼

for RF applications Rin is not a problem since people can use matching
network or transformers to tune the input impedances so that optimal power
gain can be achieved. Besides sometimes people use the 1/gm property of CG
stage to form a wideband input match to 50Ω.
In low frequency designs when people just connect the input of the second
stage to the output of the first stage it is a problem if the input
impedance of the following stage is too low.

【在 c*******l 的大作中提到】

: nod. CG的Rin大概是小些

w*u2008-03-28 07:03

28 楼

据我所知，宽带放大器都用CG。因为：宽带对noise figure 不要求很好，而且CG容
易做输入匹配到50欧姆； CG频率响应比CS好，因为没有Cgd，而Cds很小。
CS用的很多，因为其输入电阻大，在低频用的很多。noise performance better than
CG.
CG 可以说是个current buffer，输入电阻低，输出电阻高
CS 是个voltage buffer，因为输入impedance 高，输出电阻和CG差不多。