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High frequency analysis of FET
1. Common source amplifier at high frequencies:
Input capacitance (Miller Effect):
This increase in input capacitance Ci over the capacitance from gate to source is called Miller effect.
This input capacitance affects the gain at high frequencies in the operation of cascaded amplifiers. In cascaded amplifiers, the output from one stage is used as the input to a second amplifier. The input impedance of a second stage acts as a shunt across output of the first stage and Rd is shunted by the capacitance Ci.
From above figure, the output impedance is obtained by looking into the drain with the input voltage set equal to zero. If Vi = 0 in figure, r d ,Cds and Cgd in parallel. Hence the output admittance with RL considered external to the amplifier is given by
2. Common Drain Amplifier at High Frequencies:
Common Drain Amplifier Circuit & Small signal equivalent circuit at high
The output voltage Vo can be found from the product of the short circuit and the impedance between terminals S and N. Voltage gain is given by,
Input Admittance Yi can be obtained by applying Miller’s theorem to Cgs.
It is given by,
Output Admittance Yo with Rs considered external to the amplifier, it is given by
frequencies, output resistance Ro is given by,
3. Frequency Response of Common Source Amplifier:
consider a typical common source amplifier as shown in the above figure.
From above figure, it shows the high frequency equivalent circuit for the given amplifier circuit. It shows that at high frequencies coupling and bypass capacitors act as short circuits and do not affect the amplifier high frequency response. The equivalent circuit shows internal capacitances which affect the high frequency response.
Using Miller theorem, this high frequency equivalent circuit can be further simplified as follows:
The internal capacitance Cgd can be splitted into Cin(miller) and Cout(miller) as shown in the following figure.
From simplified high frequency equivalent circuit, it has two RC networks which affect the high frequency response of the amplifier. These are,
· Input RC network
· Output RC network
Input RC network:
Fig. Input RC network
From above figure,
This network is further reduced as follows since Rs<< RG
Fig. Reduced input RC network
The critical frequency for the reduced input RC network is,
Output RC network:
Fig. Output RC network
The critical frequency for the above circuit is,
It is not necessary that these frequencies should be equal. The network which has lower critical frequency than other network is called dominant network.
The phase shift in high frequency is
Determine the high frequency response of the amplifier circuit shown in the following figure.
Before calculating critical frequencies it is necessary to calculate mid frequency gain of
the given amplifier circuit. This is required to calculate Cin(miller) and Cout(miller).
Av = -gmRD
Here RD should be replaced by RD || RL Av=
Cgs = Ciss – Crss = 4pF
Now analyze the input and output network for critical frequency,
The above analysis shows that the output network produces the dominant higher critical frequency. High frequency response of the given amplifier is shown in the following figure
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