Transcript MOS Amplifiers

1
Music for your ears
9/14
Musique 101
9/14
Audio Spectrum
9/14
4
Amplifiers have limited bandwidth
 frequency response of Amplifiers
Constant gain between w1 & w2 (Bandwidth) ; otherwise lower
gain. Amp chosen so its BW coincides with required spectrum to
be amplified … otherwise signals distorted
Figure 1.21 Typical magnitude response of an amplifier. |T(v)| is the magnitude of the amplifier transfer function—
that is, the ratio of the output Vo(v) to the input Vi(v).
9/14
5
Voltage Amplifier - Transfer characteristic
output
Can also have circuit Amps
input
Figure 1.11 (a) A voltage amplifier fed with a signal vI(t) and connected to a load resistance RL. (b)
Transfer characteristic of a linear voltage amplifier with voltage gain Av.
9/14
6
MOSFET AMP
common source
Graphical method
Using load line :: slope is 1/RD
For any value of VI = VGS
Locate corresponding ID – VDS
curve
Find vo
Figure 4.26 (a) Basic structure of the common-source amplifier. (b) Graphical construction to determine
the transfer characteristic of the amplifier in (a).
9/14
7
MOSFET Amp. Biased
at Q
Xfer characteristic
 inverting
Similar to switch characteristic
Biased at point Q
Vi superimposed on VIQ
Vi small  linear amp operation
 VO proportional to Vi
Figure 4.26 (Continued) (c) Transfer characteristic showing operation as an amplifier
biased at point Q.
9/14
8
MOSFET as linear Amp – use saturation seg.
How to select Bias point
Q1 bias point
• Not enough +ve swing
• Too close to Vdd
Q2 bias point
• Too close to triode
• Insufficient –ve swing
Figure 4.27 Two load lines and corresponding bias points. Bias point Q 1 does not leave sufficient room
for positive signal swing at the drain (too close to VDD). Bias point Q2 is too close to the boundary of the
triode region and might not allow for sufficient negative signal swing.
9/14
9
Some Amplifiers Require 2 supplies
Eg 2 supply +ve & -ve swings
Figure 1.12 An amplifier that requires two dc supplies (shown as batteries) for operation.
9/14
10
Amplifier
L+ >= A*vi
transfer
characteristics
output
Amps have
limitations .. may
saturate ..signal
(2)
Amp linearity desired
Vout(t) = A * Vin(t)
input
Figure 1.13 An amplifier transfer characteristic that is linear except for output saturation.
Two power supplies used
9/14
11
Amplifier Biasing – ensures linearity
Nonlinear response
Figure 1.14 (a) An amplifier transfer characteristic that shows considerable nonlinearity. (b) To obtain linear operation
the amplifier is biased as shown, and the signal amplitude is kept small. Observe that this amplifier is operated from a
single power supply, VDD.
9/14
12
Inverting Amplifier biasing example
Top limit = 10v, lower limit = 0.3v
Output 180 degrees out of phase with input
L+ =~ 10v
@ Vt = 0
For 5 V bias
@ Vo = 5v
Vt = 0.673
L- = 0.3v
Vt = 0.690
Figure 1.15 A sketch of the transfer characteristic of the amplifier of Example 1.2. Note that this
amplifier is inverting (i.e., with a gain that is negative).
9/14
13
Voltage Amplifier Circuit Model
Inp. resistance
Out. resistance
used for simulation, circuit analysis
Inp. source, / Rs
Load RL
Gain = Aw;
input resistance = Ri, Output resistance = Ro
vo = Aw * vi * RL/(RL + Ro) ; effect of output resistance Ro
vo / vi = Aw * RL/(RL + Ro) ; voltage gain
vi = vs * Ri / (Ri + Rs)
; effect of Ri
vo / vs = Aw * Ri / (Ri + Rs) * RL/(RL + Ro); overall voltage gain
accounting for input / output impedances
Figure 1.17 (a) Circuit model for the voltage amplifier. (b) The voltage amplifier with input signal source and load.
9/14
14
More Gain ? Use Cascaded stages Ex 1.3
Input stage needs high input impedance
Output stage needs low output impedance
Input resistance of a stage = load resistance of previous stage
Vi1 / Vs = 1 M / ( 1M + 100k) = 0.909
Av1 = Vi2 / Vi1 = 10 * 100k / (100k + 1k) = 9.9
Av2 = Vi3 / Vi2 = 100 * 10k / (10k + 1k) = 90.9
Av3 = VL / Vi3 = 1 * 100 / ( 100 + 10) = 0.909
Av = VL / Vi1 = 9.9 * 90.9 * 0.909 = 818
VL / Vs = 818 * 0.909 = 743.6
Ideal Gain = 10 * 100 = 1000
Figure 1.18 Three-stage amplifier for Example 1.3.
9/14
; effect of 1st Rin
; gain 1st stage, A = 10
; gain 2nd stage, A = 100
; gain 3rd stage, A = 1
; 3 stage gain
; from source to load
15
Amplifiers have limited bandwidth
 frequency response of Amplifiers
Constant gain between w1 & w2 (Bandwidth) ; otherwise lower
gain. Amp chosen so its BW coincides with required spectrum to
be amplified … otherwise signals distorted
Figure 1.21 Typical magnitude response of an amplifier. |T(v)| is the magnitude of the amplifier transfer function—
that is, the ratio of the output Vo(v) to the input Vi(v).
9/14
16
Capacitively Coupled
Amplifier Stages
Figure 1.27 Use of a capacitor to couple amplifier stages.
9/14
17
An NMOS Common Source Amplifier
Amp. Circuit
Xfer Characteristic
Figure 4.26 (a) Basic structure of the common-source amplifier. (b) Graphical construction to determine
the transfer characteristic of the amplifier in (a).
9/14
18
NMOS common
source Amp. Cont’d
Xfer Characteristic
Biased at point Q
Figure 4.26 (Continued) (c) Transfer characteristic showing operation as an amplifier biased at point Q.
9/14
19
Pspice Amplifier Example
Figure 4.63 Capture schematic of the CS amplifier in Example 4.14.
9/14
20