1. Amplifier models Voltage controlled voltage source (VCVS)
Avo is the open circuit voltage gain
Current controlled current source (CCCS)
Aisc is the short circuit current gain
Note: Here R0 accounts for the change in output voltage or current as the load changes

2. Conversion from VCVS to CCCS
Thus a VCVS can be converted to a CCCS by first finding the current gain Aisc and then putting the series output resistance R0 in case of VCCS parallel to the current source.

3. Examples on conversion
For a voltage amplifier, if Ri = 1 K, Av0 =100, and R0 = 100 , then draw the equivalent current amplifier circuit.
Solution:
The current gain Aisc is calculated as Av0Ri/R0 = 1000
The output resistance remains the same but now is in parallel.
Input resistance is the same.
Thus, the overall circuit looks like shown at the side.
Reverse conversion from voltage to current amplifier:
Consider a current amplifier of Ri = 1 k, R0 = 20 , and short circuit current gain = 200. Convert into an equivalent voltage amplifier circuit
Solution:
The voltage gain Avc is calculated as AiscR0/Ri = 4
The output resistance remains the same but now is in series.
Input resistance is the same.

4. Transconductance and transresistance amplifier models
Transconductance amplifier model: Input is voltage, output is current
Short circuit transconductance gain
Transresistance amplifier model: Input is current, output is voltage

5. Example
Determine the transconductance and transresistance amplifier models for the amplifier shown in the above figure.

6. Summary of different types of amplifiers
So, given the basic voltage amplifier model, you should be able to determine all the other models

7. Practical applications of different i/p and o/p impedances 1
Variable input impedance of the human body. This happens mainly due to the change in conditions of the skin
High input impedance of Electro-cardiograph is necessary
The voltage across the ECG machine is: Vs x Ri/(Ri + Rs), which is to be measured.
Therefore, the higher the input impedance Ri compared to Rs, the lesser is the variation in input voltage

8. Practical applications of different i/p and o/p impedances 2
Low input resistance of the ammeter is necessary (so as to not load the circuit, and reduce the current that it is trying to measure).
In summary:

9. Practical example of low o/p impedance of the source
Audio amplifiers in various rooms of an office building:
If the amplifier feeds several loudspeakers then for different switching conditions, the load will be different for the amplifier, and the sound intensity will be different, which is undesirable.
However, if the amplifier output impedance R0 is much less than the (lowest) load resistance, the load voltage is nearly independent of the number of amplifiers turned on or off.

10. 1.9 Ideal Amplifiers

11. Basic amplifier concepts
1.10 Amplifier Frequency Response
Any periodic signal can be broken down into sinusoidal components by Fourier analysis

12. Basic amplifier concepts
An amplifier good for a particular frequency range should linearly amplify the signal (containing multiple frequencies) in that range.

13. Basic amplifier concepts
Complex Gain:
Q: What is a Phasor?
A Phasor is an ac voltage or current that maintains the same phase with respect to a standard reference. Thus, in a circuit the quantities can have a constant phase relationship independent of time.

14. Basic amplifier concepts