63.5.1. Cascode Amplifier Circuit Description

Welcome back to our online NPTEL certification course on Analog Electronic Circuit. Myself, Pradip Mandal from E and EC department of IIT Kharagpur. Today we are going to talk about Multi Transistor Amplifiers; namely, in fact this is continuation of our previous lectures where we discussed about cascode amplifiers.

The introduction sets the stage for the lecture on cascode amplifiers, which are a type of multi-transistor amplifier. These amplifiers combine the strengths of both the common emitter and common base configurations to achieve better performance in terms of gain and frequency response. They effectively improve the input and output impedance, making them more versatile for various applications in electronic circuits.

Think of a cascode amplifier like a relay team in a race, where each runner has their strengths at different segments of the race. The first runner (common emitter) starts strong, while the second runner (common base) has the speed to finish strong. By combining their strengths, the team achieves a better overall timing, similar to how a cascode amplifier optimizes electronic signal processing.

This circuit, the cascode amplifier we already have seen before. Today we are giving you numerical value of different bias components, supply voltage, and then the device parameters are given here; namely, Early voltage of both the transistors, we are assuming it would be 100 V; β or transistor-1, it is 100.

Here, we provide specific values for the parameters used in the cascode amplifier circuit. The Early voltage is an important parameter that affects the output resistance of a transistor. The transistor current gain, beta (β), indicates how effectively the transistor can amplify current. In this circuit, it’s essential to understand how these parameters influence the performance of the amplifier.

Imagine tuning a musical instrument; the Early voltage is like setting the right pitch, ensuring that the sound produced is clear and resonant. Similarly, the beta value is like the volume knob, allowing the musician to control how loud the performance is, thus affecting the overall output in an electronic circuit.

We first find the operating point of both the transistors and then we will go for the small signal parameters. Supply voltage is 12 V and then we do have R which is 570 kΩ and then we do have the V_BE which is approximately 0.6 V drop. From that we can calculate what is the I_B.

The operating point is a critical aspect of transistor performance, determining how the transistors function within the amplifier circuit. In this segment, we calculate various currents and voltages needed to establish this point. By understanding the supply voltage, resistor values, and V_BE drop, we can derive the base current (I_B), which directly impacts the collector current and overall function of the amplifier.

Think of the operating point like balancing your weight on a seesaw. You need to find just the right spot to maintain stability. If the weight is too far to one side (like having an incorrect voltage drop), it will tip over, just as a circuit might become unstable if the operating point is not set correctly.

Let us see what will be the value of small signal parameters namely g_m, then r_pi and r_o of the two transistors. First, we can say that g_m, corresponding to transistor-1 is calculated based on the collector current.

Small signal parameters are crucial for analyzing the behavior of the amplifier under varying signals. The transconductance (g_m) represents how effectively the input voltage changes affect the output current, while r_pi and r_o are the small-signal equivalent resistances. These values help in designing circuits that work effectively at the desired frequencies.

Consider a car's accelerator; g_m is like the sensitivity of how pressing the accelerator pedal affects the car's speed. If the car accelerates smoothly with minor pedal changes, it represents a high transconductance. Similarly, r_pi and r_o act like the friction of the road, affecting how easily the car can gain speed.

What is the voltage gain? So, try to remember these values of the small signal parameters to get the voltage gain. The output voltage v_o is calculated based on the input voltage and the resulting small signal currents.

In this section, we focus on calculating the overall voltage gain of the cascode amplifier using the small signal parameters we previously derived. The voltage gain represents the ratio of the output voltage to the input voltage, indicating how much the amplifier boosts the signal.

Using the analogy of a microphone at a concert, the microphone picks up a soft sound (input voltage) and then the amplifier increases its output to be loud enough for the audience to hear (output voltage). The voltage gain tells us how effectively the microphone translates the soft sound into a strong sound that fills the venue.

So, the input capacitance is considered from the base looking at the input circuit, including effects from all capacitors in the amplifier circuit. The unique setup significantly reduces the capacitance and enhances bandwidth.

The input capacitance plays a vital role in the amplifier's performance, particularly at high frequencies. In a cascode amplifier, the arrangement typically leads to a lower input capacitance, which positively impacts the bandwidth of the amplifier, allowing it to operate effectively across a wider range of frequencies.

Think about a highway compared to a small side road; a wider highway allows more vehicles (signals) to travel quickly at once (higher bandwidth). Similarly, having a lower input capacitance means that the cascode amplifier can handle higher frequencies more effectively without signal loss or distortion.

If I compare the performance; namely the voltage gain and the upper cutoff frequency for a standard CE amplifier, where we can probably, we can eliminate this part and we can directly connect the collector part...

In this comparison, we examine the performance of the cascode amplifier against the common emitter (CE) amplifier. We note that while both can achieve similar voltage gains, the cascode amplifier provides better bandwidth and stability due to its different topology. The discussion leads to understanding the specific advantages of using a cascode configuration in many applications.

Imagine two types of delivery services: one that has a reliable car (CE amplifier) and one that uses a cargo van designed for longer distances (cascode amplifier). Both can make deliveries, but the cargo van can transport heavier loads faster over longer distances, indicating that the cascode amplifier performs better in specific scenarios.

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