EECS 105 Spring 2004, Lecture 24
Lecture 24:
Single stage amplifiers
Prof J. S. Smith
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Context
In today’s lecture, we will discuss
general amplifier topology, and how
to use transistors to make single
stage amplifiers of various kinds
Department of EECS
University of California, Berkeley
1
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Reading
For about a a week after spring break, we continuing
in chapter 8 in the text, single stage amplifiers
There is no homework assignment over spring break.
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Lecture Outline
z
z
Review: Small Signal Analysis
Two Port Circuits
–
–
–
–
z
Department of EECS
Voltage Amplifiers
Current Amplifiers
Transconductance Amps
Transresistance Amps
Example: MOS Amp Again!
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Small Signal Analysis
z
Step 1: Find DC operating point. Calculate
(estimate) the DC voltages and currents (ignore
small signals sources)
z
Substitute the small-signal model of the
MOSFET/BJT/Diode and the small-signal models
of the other circuit elements.
z
Solve for desired parameters (gain, input
impedance, …)
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Amplifier Terminology
z
z
z
z
z
Sources: Signal, its source resistance, and bias
voltage or current
Load: Use resistor in Chap. 8, but could be a
general impedance
Port: A pair of terminals across which a voltage
and an associated current are defined
Source, Load: “one port”
Amplifier: “two port”
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University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
One-Port Models (EECS 40)
z
A terminal pair across which a voltage and
associated current are defined
iab
+
vab
Circuit
Block
−
iab
+
vab
iab
Rthev
vthev
+
vab
Rthev
ithev
−
−
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Small-Signal Two-Port Models
iin
z
z
+
vin
+
vout
−
−
We assume that input port is linear and most of the
time, that the amplifier is unilateral:
–
z
iout
Output depends on input but input is independent of
output.
Output port: depends linearly on the current and
voltage at the input and output ports
Unilateral assumption is good as long as “overlap”
capacitance is small (MOS)
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University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Math 54 Perspective
Can write linear system of equations for either iout or vout
in terms of two of iin, vin, iout, or vout: possibilities are
iout = α1vin + α 2 vout
iout = α 3iin + α 4 vout
vout = α 5vin + α 6iout
vout = α 7iin + α8iout
What is physical meaning of α1? of α6?
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
EE Perspective
z
Four amplifier types: determined by the output
signal and the input signal … both of which we
select (usually obvious)
–
–
–
–
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Voltage Amp (VÆV)
Current Amp (IÆI)
Transconductance Amp (VÆI)
Transresistance Amp (IÆV)
We need methods to find the 6 α parameters for the
four models and equivalent circuits for unilateral
two ports
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University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Two-Port Small-Signal Amplifiers
Rout
Rs
+
vin
vs
Rin
Av vin
RL
−
Voltage Amplifier
iin
is
Rs
Rin
Ai iin
Rout
RL
Current Amplifier
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Two-Port Small-Signal Amplifiers
Rs
+
vin
vs
Rin
Gm vin
RL
Rout
−
Transconductance Amplifier
Rout
iin
is
Rs
Rin
Rmiin
RL
Transresistance Amplifier
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Measuring the small signal parameters
The meaning of the small signal parameters can be
made more clear by setting up measurements of
them.
z For a single port model, we just need to set the
voltage, and measure the currents, or set the curent,
and measure the voltage and we will get a linear
relationship
or
I =V / R
V = RI
But for a two port model, we need to decide what to
hold constant, what to vary, and what to measure
z
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Input Resistance Rin
Looks like a Thevenin resistance measurement, but note that the
output port has the load resistance attached (for a unilateral device,
The output resistance has no effect.)
Rin =
vt
it
RS removed ,
RL attached
OR
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Output Resistance Rout
Looks like a Thevenin resistance measurement, but note that the
input port has the source resistance attached, (for a unilateral
device the input resistance won’t have an effect)
Rout =
vt
it
RL removed ,
RS attached
OR
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Finding the Voltage Gain Av
Key idea: the output port is open-circuited and the
source resistance is shorted
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Measuring Av
By setting Rs = 0, ⇒ vin = vs
By setting RL = ∞, ⇒ vout = Av vs
Rout
Rs
+
vin
vs
Rin
Av vin
RL
−
Voltage Amplifier
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Finding the Current Gain Ai
Key idea: the output port is shorted and the source
resistance is removed
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Finding the Transresistance Rm
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Finding the Transconductance Gm
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University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Common-Source Amplifier (again)
How to isolate DC level?
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
DC Bias
5V
Neglect all AC signals
2.5 V
Choose IBIAS, W/L
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Load-Line Analysis to find Q
I RD =
VDD − Vout
RD
Q
ID =
slope =
5V
10k
ID =
1
10k
0V
10k
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Small-Signal Analysis
Rin = ∞
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Two-Port Parameters:
Find Rin, Rout, Gm
Generic Transconductance Amp
Rs
vs
+
vin
Rin
Gm vin
RL
Rout
−
Rin = ∞
Gm = g m Rout = ro || RD
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Two-Port CS Model
Reattach source and load one-ports:
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Common-Source Amplifier
Isolate DC level
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Maximize Gain of CS Amp
Av = − g m RD || ro
z
z
z
Increase the gm (more current)
Increase RD (free? Don’t need to dissipate extra
power)
Limit: Must keep the device in saturation
VDS = VDD − I D RD > VDS , sat
z
z
For a fixed current, the load resistor can only be
chosen so large
To have good swing we’d also like to avoid getting
to close to saturation
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Current Source Supply
z
z
Solution: Use a
current source!
Current independent
of voltage for ideal
source
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
CS Amp with Current Source Supply
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Load Line for DC Biasing
Both the I-source and the transistor are idealized for DC bias
analysis
Department of EECS
University of California, Berkeley
EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
Two-Port Parameters
From current
source supply
Rin = ∞
Gm = g m
Rout = ro || roc
Department of EECS
University of California, Berkeley
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EECS 105 Spring 2004, Lecture 24
Prof. J. S. Smith
P-Channel CS Amplifier
DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP
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University of California, Berkeley
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