BASIC FET AMPLIFIER CONFIGURATIONS -...

Topic 4

BASIC FET AMPLIFIER

CONFIGURATIONS

AV18-AFC ANALOG FUNDAMENTALS C

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Overview

This topic identifies the basic FET amplifier configurations and their principles of operation.

ANALOG FUNDAMENTALS C AV18-AFC

2 29 Sep 09

Topic Learning Outcome

LO 4 Describe the operation of Field Effect Transistor (FET) amplifiers.

Assessment Criteria

LO 4.1 Identify the following FET amplifier configurations:

LO 4.1.1common source,

LO 4.1.2common drain, and

LO 4.1.3common gate.

LO 4.2 Describe the characteristics of the following FET amplifier configurations:




LO 4.3 Describe the operation of the following FET amplifier configurations:





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FET Amplifier Configurations

When using FETs as amplifiers, the input signal is applied across two terminals of the FET and the output is taken across two terminals.

Figure 4—1 illustrates the three FET amplifier configurations:

• common source,

• common gate, and

• common drain.

Figure 4—1Fet Amplifier Configurations


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FET Amplifiers

Common Source Amplifier

The Common Source (CS) amplifier is the FET equivalent of the common emitter transistor amplifier configuration. Like the CE amplifier, it is capable of high voltage gain.

The CS amplifier has the input applied between the gate and source terminals and the output signal taken across the drain and source terminals.

Therefore, the source terminal is common to both the input and output signals.

Figure 4—2 illustrates a CS amplifier utilising self bias.

Figure 4—2CS Amplifier-Self Bias

Note in Figure 4—2 the use of coupling capacitors CIN and COUT. Their function is to block the DC or bias current from entering the AC signal source and load resistor RL.


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Conversely, during AC operation they couple the AC signal source to the input of the amplifier and the resulting output to the load resistor. Bypass capacitor C1 is used to bypass the source resistor and increase the amplifier's voltage gain.

AC Operation

Figure 4—3 shows the self bias CS amplifier used previously, with the load resistance removed. Note the injection of an AC signal at the gate of the FET.

Figure 4—3CS Amplifier

With a BJT amplifier, an increase in the input signal resulted in an increase in the base current. A FET amplifier however, does not have any current flowing through the gate source junction.

This is due to the reverse biased PN junction (JFETs) or the silicon dioxide insulating layer (MOSFETs).

Applying an input signal has an amplitude of 2Vpk-pk.

As the input signal goes more positive, the signal current flows through the gate resistor RG. This current flow develops a voltage drop across RG which equals the amplitude of the applied voltage ie VIN = VG.


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With the gate voltage increasing from the quiescent condition (zero volts), the gate source voltage must be decreasing because the source voltage is a positive voltage. Remember, for an N channel JFET, VGS must be negative.

A decreased or less negative VGS applied to the N channel JFET (utilised in Figure 4—3), results in the drain current increasing from the quiescent condition. Note if VGS goes more negative ie -2 V to -4 V, VGS is said to be increasing.

An increasing drain current produces a larger voltage drop across the drain resistor, resulting in less voltage at the output or drain terminal.

With the input signal returning to zero volts, the FET amplifier is again at the quiescent point.

As the input signal goes more negative the process reverses, resulting in an increasing VGS and decreasing drain current.

Figure 4—4 shows the drain characteristic curve of the CS amplifier illustrating the resulting signal swings.

Figure 4—4CS Amplifier-Drain Characteristic Curve

From the operation of the CS amplifier, notice that the output waveform is 180° out of phase with the input waveform.


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Figure 4—5 shows the circuit waveforms for the common source amplifier.

Figure 4—5CS Amplifier-Waveforms

Figure 4—6 shows a simplified AC equivalent circuit for the self biased CS amplifier shown in Figure 4—2.

Figure 4—6CS Amplifier-Ac Equivalent Circuit


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Input Impedance

The input impedance is the impedance ‘seen’ by the applied voltage source looking into the amplifier.

For a FET device, the gate is insulated from the drain source channel. Therefore, the input to the FET can be treated as an open circuit.

Referring to Figure 4—7, the input impedance for the self biased CS amplifier is simply the gate resistor RG.

Figure 4—7CS Amplifier-Input Impedance

The input capacitance of a FET is larger than that of a BJT and therefore the input frequency is a factor.

For high operating frequencies, the input capacitance will effectively be in parallel with the gate resistor RG , as shown in Figure 4—8.

Figure 4—8CS Amplifier


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This capacitive reactance will act as a short circuit at high frequencies and attenuate the input signal, decreasing the overall gain of the amplifier.

Output Impedance

The output impedance is the impedance ‘seen’ by the load resistor RL.

Figure 4—9 illustrates the output impedance for the CS amplifier.

Figure 4—9CS Amplifier Output Impedance

From Figure 4—9, it can be shown that the output impedance is equal to:

∴ because rds is very high ΖOUT ≅ RD

Current Gain

For a FET, we have stated that a change in the gate source voltage (VGS) produces a change in the drain current (ID). A FET does not have an input current.

The FET equivalent of a transistor's β is known as the transconductance (gm) of the device.


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What is Transconductance?

To describe what transconductance is, Ohm's Law needs to be revisited.

Ohm's Law:

Where:

Rearranging Ohm's Law gives the equation for resistance:

What is Resistance?

Resistance as the ability of any circuit device (not just resistors) to oppose the flow of current through a circuit.

From the opposite point of view there is the circuit’s ability to accept current instead of opposing it. This is known as conductance.

This shows that conductance is basically the opposite of resistance, and the relationship can be shown as follows:

That is, as resistance increases, conductance decreases and vice-versa.

Conductance is represented by the symbol 'G' and is measured in Siemens (S).

Using Ohm's Law the formula for conductance can be derived as follows:

and


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Therefore:

Rearranging this gives:

This can now be applied to the FET amplifier.

For a FET, the output current is controlled be the gate-source voltage. Therefore, the gain must be measured in terms of:

Notice the I/V relationship which represents conductance.

In amplifier terminology, the gain is known as transconductance and is denoted by the symbol ‘gm’.

The gain is given by:

Figure 4—10 shows the drain characteristic curve of a FET illustrating the transconductance of the device. Note that the transconductance is determined at a set drain source voltage VDS.


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Figure 4—10CS Amplifier-Drain Characteristic Curve

Voltage Gain

Voltage gain is determined by the ratio of the output voltage to the input voltage.

It is important to understand that, for a FET amplifier, the drain current that flows for a given input voltage is determined by the transconductance of the device multiplied by the gate source voltage VGS.

Remember, transconductance is the amount of change (or gain) that results in the drain current for a change in the gate source voltage. The negative sign indicates a phase inversion.

For the fixed bias CS amplifier, the gate source voltage Vgs will equal the voltage in Vin.

Therefore, the voltage gain of a CS amplifier is:

An approximate expression for the common source voltage gain is:

, where Rout = RD RL

Typical voltage gains of a common source amplifier range from 20-50.


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CS Amplifier Summary

Summarising the common source amplifier:

• The output wave form is 180° out of phase with the input wave form.

• The source wave form is in phase with the input wave form.

• Voltage gain greater than unity (typically 20-50).

• High input impedance (typically 1-5 MΩ).

• Relatively high output impedance (typically 5-10 kΩ).


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Practical Exercise

Common Source Amplifier

Overview

The following practical exercises will reinforce the theory on common source amplifiers and will form part of your performance assessment for this module.

Procedure

Your Instructor will nominate which of the following Lab-Volt practical exercises you are to carry out:

1 FET Fundamentals, JFET Amplifier Exercise 2

2 FET Fundamentals, Dual Gate MOSFET Exercise 2


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Equipment

LabVolt Classroom Equipment


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Common Drain Amplifier/ Source Follower

The common drain (CD) amplifier is the FET equivalent of the common collector transistor amplifier.

The CD amplifier is also called a source follower.

Figure 4—11 illustrates a CD amplifier.

Figure 4—11Common Drain Amplifier


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AC Operation

The AC operation of the CD amplifier is the same as that of the CS amplifier. The only difference is that the drain resistor RD is removed and the drain terminal is connected directly to VDD.

The biasing configuration used in Figure 4—11 is the voltage divider.

With a signal applied at the gate, the applied signal will be summed with the bias potential established at the gate by the biasing resistors RG1 and RG2.

For example, assuming that the gate biasing potential is 2 V and the source biasing potential is 4 V, the gate source voltage is therefore:

If the applied input signal had an amplitude of 2 Vpk-pk, during the positive half cycle the positive going input signal will increase the gate potential to a peak of 3 V. This increase in the gate potential will decrease the gate source voltage.

This thereby allows more drain current to flow.

An increase in the drain current increases the voltage drop across the source resistor RS. With this increase the voltage at the source terminal also increases.

For the negative half cycle, a negative going input signal decreases the gate voltage, thereby increasing the gate source voltage.

An increase in VGS results in a decrease in the drain current. A decreasing drain current produces a smaller voltage at the source terminal.

As for the common collector transistor amplifier, the output wave form of the common drain FET amplifier is in phase with the applied input wave form.


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Figure 4—12 illustrates the circuit wave forms for the common drain amplifier.

Figure 4—12CD Amplifier-Waveforms


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The drain and source terminals are identified to enable a reference with the AC equivalent circuit.

A simplified AC equivalent circuit for the common drain amplifier is drawn by replacing the power supply and capacitors with their short circuit equivalents.

Figure 4—12 illustrates the AC equivalent circuit for the CD amplifier shown in Figure 4—13 (Note the changed orientation of the FET Source and Drain terminals).

Figure 4—13CD Amplifier-Ac Equivalent

RG1 and RG2 have been combined into the equivalent parallel resistance RGATE.


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Input Impedance

For the common source amplifier, the bias resistances constitute the input impedance for the common drain amplifier.

The input impedance is therefore:

Figure 4—14 illustrates the input impedance of the CD amplifier.

Figure 4—14CD Amplifier- Input Impedance


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Output Impedance

The common drain amplifier has a very low output impedance, as does its transistor equivalent, the common collector transistor amplifier.

In Figure 4—15 the output impedance is simply the resistance of the drain source junction of the FET.

As Rs is far greater than rsd the output impedance can be stated as:

Figure 4—15CD Amplifier-Output Impedance

Typical values of the output impedance for the CD amplifier range from 100 - 200Ω.


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Voltage Gain

For the common drain circuit, the voltage gain can be determined by:

(without load)

∴ remember

Substitute into the original equation:

ie. AV < 1

Looking at the final voltage gain equation for the CD amplifier, you can see that the top line of the equation will always be less than the bottom line of the equation. If gmRS is very large (as is normally the case) the voltage gain of the CD amplifier is approximated to one as was the case for the common collector amplifier.

However: with a load.


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CD Amplifier Summary

Summarising the common drain amplifier:

• The output wave form is in phase with the input wave form.

• Voltage gain is less than unity (typically 0.98).

• High input impedance (typically 1-5 MΩ).

• Extremely low output impedance (typically 100-200Ω).

The common drain amplifier provides a voltage gain close to unity. The high input impedance and low output impedance enables the CD amplifier to be utilised as a buffer stage between two differing loads.

It is also known as a source follower amplifier.


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Common Gate Amplifier

The common gate (CG) amplifier is the FET equivalent of the common base transistor amplifier.

In the common gate amplifier, the input is applied to the source terminal and the output is taken from the drain terminal. Note that the gate terminal is grounded.

Figure 4—16 illustrates a CG amplifier.

Figure 4—16Common Gate Amplifier


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AC Operation

With no input signal, the DC bias current will establish a voltage drop across the source resistor RS. As the gate is grounded, the gate source voltage VGS will be controlled by this source resistance voltage drop.

For example, if the DC quiescent source voltage is 1 V, the gate source voltage will be -1 V.

When an input signal is applied, the voltage at the source of the FET will increase or decrease. As the source voltage is varying, the gate source voltage also varies.

If a positive going input signal increased by 0.5 V, the gate source voltage will increase to -1.5 V. An increase in VGS results in a decrease in the drain current.

This decrease in the drain current results in a decrease voltage drop across RD and the voltage at the drain of the FET increasing. Therefore, an increase in the applied input signal has produced an increase in the output signal.

Conversely, a negative going input signal will reduce VGS, increasing the drain current. An increase in ID will result in the output voltage decreasing.

The resulting output waveform is in phase with the input signal.


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Figure 4—17 illustrates the circuit waveforms for the CG amplifier.

Figure 4—17CG Amplifier-Waveforms

The AC equivalent circuit shown in Figure 4—18 is derived as for the previous FET amplifiers.

Figure 4—18CG Amplifier-Ac Equivalent


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Input Impedance

Figure 4—19 shows the input impedance of the common drain amplifier.

Figure 4—19CG Amplifier- Input Impedance

As discussed in CD amplifiers, ; so does rsg.

The input impedance of the CG amplifier is therefore:

From this it can be seen that the source gate resistance is equal to the inverse of the transconductance of the FET. The value of rsg is very small, typically 100Ω.


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Output Impedance

The output impedance for the CG amplifier is shown in Figure 4—20.

Figure 4—20CG Amplifier-Output Impedance

The output impedance is approximately:

since rdg is usually very high compared to RD:

Typical values for the output impedance of the CG amplifier are approximately 5 kΩ.


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Voltage Gain

The voltage gain for the common gate amplifier is derived in a similar manner as that of the common drain amplifier.

The output voltage of the CG amplifier can be determined as:

Therefore, the voltage gain is approximately:

The absence of a minus sign indicates that the voltage output is in phase with the input voltage.

Typical voltage gains of the common gate amplifier range from 20-50.

CG Amplifier Summary

Summarising the common gate amplifier:

• The output waveform is in phase with the input wave form.

• Voltage gain greater than unity (typically 20-50).

• Extremely low input impedance (typically 100-200Ω).

• Relatively high output impedance (typically 5-10 kΩ).

The common gate amplifier provides good voltage gain and the output waveform is in phase with the input waveform.


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Trainee Activity

1. What is transconductance?

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2. Draw the AC equivalent circuit for the above circuit diagram.


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3. For the above circuit, determine the following:

A. AV

B. ZIN

C. ZOUT

D. gm

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4. Draw the AC equivalent circuit for the above circuit diagram.

5. What type of amplifier is the circuit shown in Question 4?

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6. Which FET amplifier configuration has a voltage gain less than unity?

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7. For a common source amplifier, the output waveform is in phase/out of phase with the applied input waveform.

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8. What type of amplifier configuration is the above circuit?

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9. Draw the AC equivalent circuit for the circuit shown in Question 8.

10. Describe the operation of the circuit shown in Question 8 with the applied voltage going more positive.

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End of Topic Text


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