Simple Cascode Amplifier Circuit

The cascode amplifier is a type of two stage amplifier circuit that is often found in analog electronics.

It helps to enhance important features like gain, bandwidth and input and output impedance.

By combining the strengths of both common source and common gate amplifier setups, it reduces problems such as miller capacitance and boosts stability.

You can find cascode amplifiers in many applications, including RF amplifiers, high speed operational amplifiers and instrumentation systems.

This article goes into detail about cascode amplifier circuits especially focusing on the FET based cascode amplifier and how to implement it in real life.

It also explains how it works, includes important formulas, shows how to build the circuit and offers practical tips.

Circuit Working:

Working of FET cascode amplifier circuit:

The circuit diagram of a typical Cascode amplifier using FET shows the first part of the circuit is a common source amplifier made with an FET, where the input voltage Vin is connected to its gate.

The second part is a common gate amplifier, which is powered by the first stage.

Rd represents the drain resistance in the output stage.

The output voltage Vout is taken from the drain terminal of Q2.

Since Q2s gate is connected to ground the source voltage of Q2 and the drain voltage of Q1 stay nearly constant.

This means that the upper FET Q2 provides a low input resistance to the lower FET Q1.

As a result the gain of Q1 is reduced which also lessens the Miller effect leading to a wider bandwidth.

Even though the gain of Q1 decreases the overall gain remains stable because Q2 compensates for it.

The upper FET Q2 is not influenced by the Miller effect since the charging and discharging of the stray capacitance between the drain and source happens through the drain resistor and the load affecting the frequency response mainly at high frequencies well beyond the audio range.

Formulas:

Formulas for the Basic FET Cascode Amplifier circuit is mentioned below:

Voltage Gain (Av):

Av​ = gm​ * Rd​

where,

• gm​ is the transconductance of the FET (gm = 2ID/ VGS −Vth)
• Rd is the drain resistance of Q2.

Input Impedance (Zin​):

Zin​ = Rg1​

where,

• Rg1​ is the gate resistance of Q1.

Output Impedance (Zout​):

Zout​ = Rd​

• Rd​ is the drain resistance of Q2.

Bandwidth (BW):

Bandwidth is improved because the Miller effect is minimized:

BW ∝ ​1​ / Cgd​ * Av

where,

• Cgd​ is the gate drain capacitance.

Working of realistic cascode amplifier circuit:

Parts List:

A realistic cascode amplifier circuit that uses a FET is displayed above.

The resistors R4 and R5 create a voltage divider to provide biasing for the FET Q2.

R3 acts as the drain resistor for Q2 helping to control the drain current.

R2 serves as the source resistor for Q1 while C1 is the capacitor that bypasses it.

R1 makes sure that there is no voltage at the gate of Q1 when there is no signal.

Formulas:

Formulas for the realistic cascode amplifier circuit is mentioned below:

Voltage Gain (Av):

Av​ = ​gm​ * R3​​ / 1+gm​ * R2

where,

• gm​ is the transconductance of Q1 (gm = 2ID / VGS−Vth)
• R3 is the drain resistance of Q2 load resistor.
• R2 is the source resistance of Q1.

Input Impedance (Zin​):

Zin​ = R1​

where,

• R1​ is the gate resistance of Q1.

Output Impedance (Zout):

Zout​ = R3​

where,

• R3​ is the drain resistance of Q2.

Biasing Currents:

The operating point of the FETs can be determined using the DC bias resistors:

ID​ = VGS​−Vth​​ / R2​

where,

• VGS​ is the gate to source voltage.
• Vth is the threshold voltage of the FET.

Coupling and Bypass Capacitors (C1,C2​):

The capacitive reactance should be low compared to the impedance at the operating frequency:

Xc ​= 1​ / 2πfC

where,

• where R is the associated resistance.

Bandwidth (BW):

Improved due to reduced Miller effect:

BW = 1​ / 2π * Rd ​* Cgd​

where,

• Cgd is the gate drain capacitance of Q1.

How to Build:

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

• Gather all the components as mentioned in the above realistic cascode amplifier circuit diagram
• Connect a gate of FET Q1 to Vin trough resistor R1 and GND.
• Connect drain of FET Q1 to source of FET Q2.
• Connect source of FET Q1 to GND through resistor R2.
• Connect a capacitor C1 from source pin of Q1 to GND.
• Connect a gate pin of FET Q2 to positive of capacitor C2 and negative of capacitor C2 to GND.
• Connect a drain pin of FET Q2 to Vout through resistor R3 to positive supply.
• Connect the source pin of FET Q2 to drain pin of FET Q1
• Connect a resistor R4 from positive supply gate of FET Q2, connect resistor R5 from gate of FET Q2 to GND.

Conclusion:

The cascode amplifier is a smart design that helps get a lot of gain a wide range of frequencies and less interference from the Miller effect.

In its basic setup it uses both common source and common gate stages, which helps keep the signal clear and improves how it responds to different frequencies.

When you add proper biasing and coupling networks the practical cascode amplifier becomes even more stable and performs better.

This is why cascode amplifiers are great for radio frequency, high speed and precise applications where having stability and a wide bandwidth is really important.

References:

Cascode