Cascode amplifier lab experiment

Common-emitter and emitter-follower amplifiers are the most widely used single-transistor amplifiers. The common-base configuration, illustrated below in its basic NPN form, is used less frequently as a stand-alone voltage amplifier stage, mostly because it has a low input resistance, but it is often combined with a common-emitter stage to form a cascode amplifier.

In a common-base voltage amplifier, the input voltage is applied to the emitter and the output voltage is taken from the collector, and the input and output voltages are in-phase. As with all linear Class A BJT amplifiers, the transistor must operate in the forward active region in the common-base amplifier.

This means that the base-emitter junction must be forward biased, the collector-base junction must be reverse biased, and operation must be prevented from entering the saturation region. The base is often biased using a resistive voltage divider, voltage regulator, or available power supply, and an emitter resistor, R E is used to establish the emitter current.

The base should always be bypassed with a capacitor that produces a low impedance AC ground at all signal frequencies. If the amplifier is DC-coupled, the voltage source applied to the base must also provide a low DC resistance.

The collector circuit is similar to what is used in the common-emitter amplifier, and in its simplest form consists of a resistor, R C connected between the collector and the power supply.

EC8361 Syllabus Analog and Digital Circuits Laboratory Regulation 2017 Anna University

The input of a common-base amplifier looks into the emitter, which is the same as looking into the output resistance of an emitter-follower amplifier.

The gain of a common-base amplifier can be calculated using detailed circuit analysis or approximated by inspection as was done with the CE amplifier. Besides the low input resistance issue, the gain of the common-base amplifier depends on r ewhich is small producing high gain, which may not be desirabletemperature dependent, and nonlinear. The greatest advantage of the cascode amplifier, however, is that it reduces the Miller effectwhich causes the bandwidth of a CE stage to decrease as its gain increases.

The voltage gain is provided by the collector current flowing through the collector resistor in the common-base stage, and the common-base amplifier bandwidth is not diminished by the Miller effect.

The important benefit realized by the cascode configuration is that high gains can be achieved without significant bandwidth loss due to the Miller effect, but it requires two transistors. As with CE amplifiers, an emitter-follower stage is often added on the output of the common-base stage to drive low impedance loads. In this lab, we will not build a standalone common-base amplifier, but will instead study the use of the common-base amplifier in a cascode amplifier.

To understand the operating principles of common-base and cascode amplifiers. To understand how to set up the proper bias conditions for a cascode amplifier and verify that the bias voltages in the circuit are close to their designed values. Following completion of this lab you should be able to explain the basic operation of common-base and cascode amplifiers, and be able to calculate the voltage gain for each of these amplifiers.

The common-base amplifier has a very low input resistance, which is equal to the incremental emitter resistance r e. This is the same as the output resistance of the emitter-follower amplifier that has a low equivalent resistance on its base, which makes sense since we are looking into the emitter. One of the most common uses of the common-base stage is as the output stage of a cascode amplifier. The cascode amplifier is comprised of a common-emitter input stage and a common base output stage.

The common-emitter stage provides high input resistance, which is desirable for voltage amplifiers. The common-emitter stage, however, suffers from the Miller effect, which produces a reduction in amplifier bandwidth as the amplifier voltage gain is increased. If the two transistors used in the amplifiers are well-matched, the r e of the common-emitter stage will approximately equal r e of the common base stage, and the gain of the common-emitter stage will be approximately The common-base amplifier configuration does not suffer from the Miller effect, so it can provide the required voltage gain without any bandwidth penalty.

The two amplifiers together, therefore, can provide voltage gain without incurring the bandwidth reduction due to the Miller effect. The approximate overall gain of the cascode stage can be quickly determined by inspection, using a few simplifications used in the common-emitter lab. The first simplification is to ignore base currents and think of the emitter and collector currents in both transistors all being equal. Referring to the cascode amplifier schematic, we can think of the same current flowing through the path consisting of the collector and emitter of the common-base transistor into the collector and emitter of the common-emitter stage, much in the same way as the current would flow in a series circuit.

In our circuit, part of the emitter bias resistor is bypassed and part is not. The general approximate voltage gain for the cascode amplifier can now be expressed as. In the cascode amplifier studied in this lab, the collector DC bias current I C is approximately 2.The stages are in a cascode configuration stacked in series, as opposed to cascaded for a standard amplifier chain. The cascode amplifier configuration has both wide bandwidth and a moderately high input impedance.

The cascode amplifier is combined common-emitter and common-base. This is an AC circuit equivalent with batteries and capacitors replaced by short circuits.

The key to understanding the wide bandwidth of the cascode configuration is the Miller effect. The Miller effect is the multiplication of the bandwidth robbing collector-base capacitance by voltage gain A v.

This C-B capacitance is smaller than the E-B capacitance. Thus, one would think that the C-B capacitance would have little effect.

Cascode Labs

However, in the C-E configuration, the collector output signal is out of phase with the input at the base. The collector signal capacitively coupled back opposes the base signal. Moreover, the collector feedback is 1-Av times larger than the base signal.

cascode amplifier lab experiment

Keep in mind that Av is a negative number for the inverting C-E amplifier. This capacitive gain reducing feedback increases with frequency, reducing the high frequency response of a C-E amplifier. The emitter current is set to 1. A common-base configuration is not subject to the Miller effect because the grounded base shields the collector signal from being fed back to the emitter input.

Thus, a C-B amplifier has better high frequency response. To have a moderately high input impedance, the C-E stage is still desirable. The way to reduce the common-emitter gain is to reduce the load resistance. We now have a moderately high input impedance C-E stage without suffering the Miller effect, but no C-E dB voltage gain. Thus, the cascode has moderately high input impedance of the C-E, good gain, and good bandwidth of the C-B. The SPICE version of both a cascode amplifier, and for comparison, a common-emitter amplifier is shown in Figure above.

The netlist is in Table below. The AC source V3 drives both amplifiers via node 4. The bias resistors for this circuit are calculated in an example problem cascode.Volume 4 No.

Chitsaz-zadeh mchitsazzadeh ccac. The purpose of this experiment is to investigate the operation of a cascaded CE swamped transistor amplifier to compute, analyze, measure, compare, and finally summarize the results of this experiment in a laboratory report. Students should first draw the dc and ac equivalent circuit for this amplifier in order to calculate the ac emitter resistance r'einput impedance of the base, input impedance of the stage, and voltage gain for each stage.

For cascaded CE stages, the input impedance of the second stage comprises the load for the first stage. Therefore, for cascade amplifiers there is a loading effect on the preceding stage. In other words, the unloaded voltage gain is higher than the loaded voltage gain. In this experiment students will construct a cascaded CE swamped amplifier to stabilize and boost the overall voltage gain. Secondly, students will compute the unloaded and loaded voltage gain by using Thevenin's method.

Third, students will construct the circuit shown by using the breadboard and measuring voltage gain. Then students will be expected to measure voltage gain of each stage and the overall voltage gain in order to compare the measured values in actual laboratory with measured values by computer simulation and calculated values. The computer simulation can be performed with different versions of ElectronicWorkbench or other software packages, it is easier to use Electronic Workbench ,version 5 or latest version MultiSim and use the Bode Plotter to measure the dB gain.

Students will draw the ac equivalent circuit and calculate the input impedance Z in for each stage, and the unloaded voltage gain of each stage. Students will determine the final output voltage v out by using the unloaded voltage gain and T hevenin's method or by using the loaded voltage gain method.

In order to measure the ac voltage gain of the amplifier shown, students will first measure the dc voltages with no applied ac input voltage. If these are close to calculated values, then students will apply the ac input signal to the input of the amplifier, using a dual-trace oscilloscope to check the input and output signals.

If the input signal is amplified, the DMM can be used to measure applied input, signal at base of each transistor, and final output voltage.

Students should determine if the measured output signal v out is close to the calculated value? They should also determine if the output signal is in phase or out of phase with respect to the input signal? Using a jumper wire and temporarily shorting the swamping resistor R E1students will observe the effect on the output.

After opening and shorting the uF capacitor across R e1students will record their observation of the output signal? Shorting the resistors, R gR 2R c1and R L one at a time, students will explain their observations of the output signal. Load the Electronics Workbench and draw the amplifier circuit shown above. Attach the dual-trace oscilloscope to the input and output. If they observe an amplified signal, their circuit is working and is ready for measurement.

Measure the input signal at the base of each stage and the final output signal with an AC voltmeters and DMM as shown in the circuit shown above.

Use the bode plotter to measure the dB voltage gain. The figure below shows the computer simulation of the laboratory experiment; the oscilloscope shows the input and amplified output signal. The bode plotter shows the measured dB gain of the amplifier. Describe the function and purpose of each component in the circuit.Cascode amplifier is a two stage circuit consisting of a transconductance amplifier followed by a buffer amplifier.

Miller effect is actually the multiplication of the drain to source stray capacitance by the voltage gain. The drain to source stray capacitance always reduces the bandwidth and when it gets multiplied by the voltage gain the situation is made further worse.

Mulitiplication of stray capacitance increases the effective input capacitance and as we know, for an amplifier, the increase in input capacitance increases the lower cut of frequency and that means reduced bandwidth. Miller effect can be reduced by adding a current buffer stage at the output of the amplifier or by adding a voltage buffer stage before the input. The circuit diagram of a typical Cascode amplifier using FET is shown above.

The input stage of the circuit is an FET common source amplifier and the input voltage Vin is applied to its gate. The output stage is an FET common gate amplifier which is driven by the input stage.

Cascode amplifier

Rd is the drain resistance of the output stage. This reduces the gain of lower FET Q1 and as a result the Miller effect also gets reduced which results in increased bandwidth.

In Cascode configuration, the output is well isolated from the input. Q1 has almost constant voltage at the drain and source terminals while Q2 has almost constant voltage at its source and gate terminals and practically there is nothing to feed back from the output to input. The only points with importance in terms of voltage are the input and output terminals and they are well isolated by a central connection of constant voltage.

A practical Cascode amplifier circuit based on FET is shown above. R3 is the drain resistor for Q2 and it limits the drain current. R2 is the source resistor of Q1 and C1 is its by-pass capacitor. R1 ensures zero voltage at the gate of Q1 during zero signal condition.

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cascode amplifier lab experiment

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