Chapter 30
Introduction
|
|
|
|
Characteristics |
|
High input impedance |
|
Low output impedance |
|
High open-loop gain |
|
Two inputs |
|
One output |
|
Usually + and – dc power supplies |
Introduction
|
|
|
|
Ideal Characteristics |
|
zin (inverting) ≈ ∞ |
|
zin (non-inverting) ≈ ∞ |
|
zout ≈ 0 |
|
Av ≈ ∞ |
Introduction
|
|
|
|
Uses |
|
Comparators |
|
Voltage amplifiers |
|
Oscillators |
|
Active filters |
|
Instrumentation amplifiers |
Introduction
|
|
|
|
Single-ended amplifier |
|
One input grounded |
|
Signal at other input |
|
Double-ended amplifier/Differential
amplifier |
|
Signals at both inputs |
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Basic differential amplifier |
|
Q1 identical to Q2 |
|
RC1 = RC2 |
|
IC1 = IC2 and emitter currents equal |
|
Also, IC ≈ IE
for high β |
|
and VBE ≈ 0.7 V |
|
Similar calculation of Bias |
Differential Amplifier
and Common-Mode Signals
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Apply same signal to both Bases |
|
Vout = Vout1 – Vout2
≈ 0 |
|
Eliminates common-mode signals |
|
60 Hz |
|
Noise |
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Apply sinusoids to both bases: |
|
Same amplitude, 180° difference in
phase, |
|
if Vin1 = –Vin2 |
|
Vout = 2Vin |
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Common-mode signals |
|
Differential voltage gain |
|
|
|
|
|
also called open-loop voltage gain |
|
20,000 ≤ Av ≤
200,000 |
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Common-mode signals |
|
Common-mode voltage gain |
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Common-mode rejection ratio (CMRR) |
|
Equations |
|
|
|
|
|
|
|
Values |
Differential Amplifier
and Common-Mode Signals
|
|
|
|
Noise |
|
Static in audio signal |
|
Increases as signal is amplified |
|
Common mode signal |
|
Significantly reduced by differential
amplifier |
Negative Feedback
|
|
|
|
|
Op-amp |
|
Large differential, open-loop voltage
gain |
|
Avol
≈ 100,000 |
|
Small input yields saturated output (VCC
or VEE) |
Negative Feedback
|
|
|
|
Negative feedback |
|
Returns a portion of output signal to
the input |
|
Open-loop voltage gain decreased |
Negative Feedback
|
|
|
|
Input impedance still high |
|
Output impedance low |
|
Circuit voltage gain, Av |
|
Adjustable |
|
Stable |
Inverting Amplifier
Inverting Amplifier
|
|
|
|
Output 180° out of phase with input |
|
Significant decrease in gain |
|
Gain now called closed-loop voltage
gain |
|
Output impedance ≈ 0 |
|
vd ≈ 0 |
Inverting Amplifier
|
|
|
Inverting input at virtual ground, vin(-)
≈ 0 |
|
iin to op-amp ≈ 0 |
|
Input current only dependent on vin
and R1 |
|
Avcl only dependent on input
resistor and feedback resistor |
Inverting Amplifier
Inverting Amplifier
Non-Inverting Amplifier
The Non-Inverting
Amplifier
|
|
|
Very high input impedance |
|
Voltage gain related to the two
resistors |
|
Very low output impedance |
|
Excellent buffer |
Non-Inverting Amplifier
|
|
|
|
Differential voltage |
|
vd ≈ 0 |
|
Input current to op-amp |
|
i = 0 |
|
Closed-loop voltage gain (Avcl)
is a resistor ratio |
Non-Inverting Amplifier
Non-Inverting Amplifier
Non-Inverting Amplifier
Non-Inverting Amplifier
|
|
|
Very high zin |
|
Very low zout |
|
Good buffer circuit |
|
Also called voltage follower (gain = 1) |
|
Or adjustable gain > 1 |
Non-Inverting Amplifier
|
|
|
|
Voltage Follower Buffer Circuit |
|
Gain = 1 |
|
High impedance source drives low
impedance load |
Op-Amp Specifications
|
|
|
|
LM 741 series |
|
Inexpensive |
|
Widely used |
|
Good general specifications |
|
Characteristic of all op-amp
specifications |
|
Provide Minimum, Typical, and Maximum
ratings |
Op-Amp Specifications
|
|
|
|
Input Offset Voltage, Vio |
|
LM741C, Vio typical is 2
millivolts |
|
Model is voltage source with value, Vio
in series with + input |
Op-Amp Specifications
|
|
|
|
Input Offset Voltage, Vio |
|
Without feedback this would saturate
output with no input |
|
With negative feedback, output due to Vio
is closed-loop gain times Vio |
Op-Amp Specifications
|
|
|
Input Offset Current, Ios |
|
Ios = Difference between
bias currents at + and – inputs of op-amp |
|
741C typical Ios is 20
nanoamps |
|
Multiplying resistor used to measure Ios |
Op-Amp Specifications
|
|
|
|
|
Input Resistance |
|
741C: minimum = .3 MΩ, typical =
2 MΩ |
|
Open-Loop Voltage gain (Avol) |
|
741C: Avol = Large Signal
Voltage Gain |
|
minimum = 20,000, typical = 200,000 |
|
Closely related to Bandwidth, BW |
Op-Amp Specifications
|
|
|
|
Gain-bandwidth product |
|
741C = 1,000,000 = 106 MHz |
Op-Amp Specifications
|
|
|
Gain versus frequency curve for op-amp |
Op-Amp Specifications
|
|
|
|
Slew rate |
|
Maximum rate of change of output
voltage |
|
|
|
|
|
|
|
741C maximum slew rate = 0.5 V/μsec |
Op-Amp Specifications
|
|
|
Fastest time for output to go from 0 to
10 volts is 20 μsec |
|
Can distort waveforms that have too
fast a rise time |
Op-Amp Specifications
|
|
|
Slew rate required for Sinusoid with
frequency f and amplitude A |
|
Maximum amplitude of a sine wave with
frequency f for a given slew rate |
Op-Amp Specifications
|
|
|
Bias Compensation: use RC =
R1||RF |
Troubleshooting an Op-Amp
Circuit
|
|
|
|
Problems occur when circuit is first
built |
|
Most important |
|
Correct connection of dual power supply |
|
Connecting a – supply to a + input (or
vice versa) can burn out an op-amp |
|
Single earth ground |
|
Short connecting wires |