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Simple Cascode Amplifier Circuit

Cascode amplifier is 2-stage circuit which is used in analog electronics.

It gives better gain, bandwidth, input and output impedance.

Mix of common source and common gate amplifier helps reduce miller effect and gives more stability.

It is used in RF amps, fast op-amps and instruments.

This article for Simple Cascode Amplifier Circuit teaches us about FET Cascode amp how it works, formulas, real circuit making and the tips required while making.

Circuit Working:

Working of FET cascode amplifier circuit:

Cascode amp circuit with FET has two parts.

First part is common source FET with input Vin goes to its gate.

Second part is common gate FET which is powered by first stage.

Rd is drain resistor in output stage.

Vout taken from drain of Q2.

Q2 gate is at ground so Q2 source and Q1 drain stay almost at same voltage.

Q2 gives low input resistance to Q1.

This lowers gain of Q1, but also reduces Miller effect for better bandwidth.

Q2 makes up for Q1 gain loss so total gain stays good.

Q2 is not much affected by Miller effect.

Capacitance charge/discharge goes through Rd and load which affects only a very high frequencies.

Formulas:

Here are basic formulas for FET Cascode amplifier:

Voltage Gain (Av):

Av = gm × Rd

where,

Input Impedance (Zin):

where,

Zin = Rg1

Output Impedance (Zout):

Zout = Rd

where,

Bandwidth (BW):

BW goes up because Miller effect is small with following formula:

BW ∝ 1 / (Cgd × Av)

where,

Working of realistic cascode amplifier circuit:

Parts List:

Component TypeValueQuantity
Resistors (All resistors are 1/4 watt unless specified)10M1
330Ω1
3.9k1
27k1
10k1
CapacitorsElectrolytic 1µF1
SemiconductorsFET IRF5102

Real FET cascode amp circuit shown is above.

R4 and R5 make voltage divider and give bias to Q2.

R3 is drain resistor of Q2 and controls drain current.

R2 is source resistor of Q1.

C1 is bypass capacitor for R2 which improves the gain.

R1 keeps gate of Q1 at zero volts when there is no input signal.

Formulas:

Below are the formulas for the realistic FET cascode amplifier:

Voltage Gain (Av):

Av = gm × R3 / (1 + gm × R2)

where,

Input Impedance (Zin):

Zin = R1

where,

Output Impedance (Zout):

Zout = R3

where,

Bias Current (ID):

ID = (VGS − Vth) / R2

where,

Coupling/Bypass Capacitors (C1, C2):

Xc = 1 / (2πfC)

where,

Bandwidth (BW):

BW = 1 / (2π × Rd × Cgd)

where,

Note: Miller effect is small with BW is better.

How to Build:

To build a Simple Cascode Amplifier Circuit follow the below mentioned connections steps:

Conclusion:

Simple Cascode Amplifier Circuit is smart circuit.

It gives high gain and works on wide frequencies and has less problem from Miller effect.

It mixes common source + common gate and keeps signal clean which is good for high speed.

With right biasing and capacitors the circuit becomes strong and stable.

It is best for RF, fast op-amps and accurate systems where bandwidth matters.

References:

Cascode

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