The Operational Amplifier

The Operational Amplifier
Ideal amplifier
- high input impedance
- low output impedance
- infinite gain
- infinite bandwidth
- infinite common mode rejection ratio
- low noise
The two "golden rules":
I. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero.
II. The inputs draw no current.
Resistor should ideally be in the kilo ohm range
Resistor in Megaohm range can cause errors due to thermal noise and leakage currents.
Differential Amplifier:
- purpose cleaning up current on a line, noise reduction, instrumentation,
Note: R1 = Rf = R2 = Rg will not generate any gain but only amplify the difference. Using a different ratio for R1
and Rf or R2 and Rg will introduce a gain factor in the output
Task:
1. Calculate the output using the formula above:
V1 = 5V
V2 = 2V
All resistors have a value of 10k
2. Calculate the output:
with the same voltages as above but with different resistors
R1 = 10k
R2 = 10k
Rg = 20k
Rf = 20k
What do you find?
Inverting Amplifier:
Note: that the output voltage is inverted i.e. positive Voltage is applied at the input a negative Voltage will be seen
at the output
Therefore the input impedance at DC level is calculated
Zin = Vin/Iin
Non-Inverting Amplifier
Therefore the input impedance is determined by R1
Task: You have got a box full of 10k resistors, hundreds of them. You have a dynamic microphone which you
want to use for a PA system. Unfortunately the PA system has only got "Line" level input.
Using diagram below work out what a typical line level input is:
A dynamic microphone has a typical input level of 1.5 millivolts RMS.
Work out:
- peak voltage for a typical dynamic microphone
- how much gain do you need to generate to achieve 0dBu Ref level (pro)?
- Which Op-Amp design would you choose Non-Inverting or Inverting, what will be the difference
- What will be the significance if the input impedance is low?
- Design an Op Amp to reach line level voltages
- How would you use your 10k resistors to get the desired values - Series or Parallel connections
- using multisims verify your findings
- with line level input impedance of 10k, would you require a voltage follower? Discuss?
Voltage Follower
(unity gain amplifier or unity buffer amplifier)
Realistic value for Z is about 1Mohm to 1 Tohm
The circuit is used to minimise impact on the source where Vin comes from and to provide higher current at Vout
without impacting the source circuit.
Nevertheless the circuit may become unstable when connected to a capacitive load.
A solution may be to use two inverting amplifiers configuration with R1 and Rf being of equal value.
Summing Amplifier
If
R1 = R2 = R3 = Rf
then
Comparator
The slightest difference in the supply will cause the output to swing to the supply rail!
Task: At which input was the signal applied?
Where would you look for slew?
The Inverting Integrator
Remember: Capacitors like a.c. but hate DC
An ac signal can easily pass through a capacitor, the higher the frequency the lower the reactance Xc measured in
ohms.
This circuit is a low pass filter! Allow low Hz to go through freely and higher frequencies to be cut out. Or top cut
filter.
A pointer:
RC is the time constant, it will give the time it takes for a signal to reach it's peak. 95% of the peak is achieved after
three time constants.
Fc stands for the cut off frequency. At this frequency the signal will loose the 3dB point of loss!
Inverting Differentiator
Remember that capacitors hate DC and love ac. At high f the X is low and vice versa!
This circuit therefore works as a high pass filter!
Instrumentation Amplifier!
- High input impedance
- high common mode rejection
- low DC offset