Small-signal amplification and power amplification are dependent on ?the application where they are going to be applied.? In most cases, the ?frequency range of the signal is critical to its a

Small-signal amplification and power amplification are dependent on the application where they are going to be applied. In most cases, the frequency range of the signal is critical to its ability to carry the information that is being transferred.

In this activity, we will explore the effects of the internal and external capacitances on the frequency response of the system. We will also discuss the advantages and disadvantages of using the amplifier's response to eliminate unwanted noise from the system.

Before you begin, be sure you review the following resources:

In your original post, answer the following:

In your own words, explain the difference between low pass filters and high pass filters. A Common-Emitter BJT amplifier acts as what type of filter?
In your own words, explain how internal capacitance and external capacitance affects the overall filter characteristics
How does having two cut off frequencies that are the same effect the overall cut off frequency?
When you have a multi-stage amplifier, how do you determine the bandwidth of the overall frequency response of the amplifier?

Electronic Devices

10th ed.

Chapter 10

Amplifier Frequency Response

Electronic Devices

10th ed.

◆ Explain how circuit capacitances affect the frequency

response of an amplifier

◆ Use the decibel (dB) to express amplifier gain

◆ Analyze the low-frequency response of an amplifier

◆ Analyze the high-frequency response of an amplifier

◆ Analyze an amplifier for total frequency response

◆ Analyze multistage amplifiers for frequency response

◆ Explain how to measure the frequency response of an amplifier

Objectives:

Electronic Devices

Effect of Coupling Capacitors

Coupling capacitors are in series with the signal and are part of a high-pass filter network. They affect the low-frequency response of the amplifier.

Electronic Devices

Effect of Coupling Capacitors

Coupling capacitors are in series with the signal and are part of a high-pass filter network. They affect the low-frequency response of the amplifier.

The equivalent circuit for C1 is a high-pass filter:

Electronic Devices

Effect of Coupling Capacitors

Coupling capacitors are in series with the signal and are part of a high-pass filter network. They affect the low-frequency response of the amplifier.

The equivalent circuit for C1 is a high-pass filter:

Electronic Devices

Effect of Coupling Capacitors

Coupling capacitors are in series with the signal and are part of a high-pass filter network. They affect the low-frequency response of the amplifier.

The equivalent circuit for C1 is a high-pass filter:

C3 and (RC + RL) form another high-pass filter.

Electronic Devices

Effect of Coupling Capacitors

With FETs, the input coupling capacitor is almost always smaller because of the high input resistance. The output capacitor may be smaller or larger depending on the drain and load resistor size.

Electronic Devices

Effect of Coupling Capacitors

With FETs, the input coupling capacitor is almost always smaller because of the high input resistance. The output capacitor may be smaller or larger depending on the drain and load resistor size.

For the circuit shown, the equivalent low-pass filter for the input is simply C1 in series with RG because the gate input resistance is so high and can be ignored.

Electronic Devices

Effect of Bypass Capacitors

A bypass capacitor causes reduced gain at low-frequencies and has a high-pass filter response. The resistors “seen” by the bypass capacitor include RE, re’, and the bias resistors.

Electronic Devices

Effect of Bypass Capacitors

A bypass capacitor causes reduced gain at low-frequencies and has a high-pass filter response. The resistors “seen” by the bypass capacitor include RE, re’, and the bias resistors.

The equivalent high-pass filter for C2 is:

Electronic Devices

Effect of Bypass Capacitors

A bypass capacitor causes reduced gain at low-frequencies and has a high-pass filter response. The resistors “seen” by the bypass capacitor include RE, re’, and the bias resistors.

The equivalent high-pass filter for C2 is:

Question:

How would an emitter swamping resistor affect the response?

Electronic Devices

Effect of Bypass Capacitors

A bypass capacitor causes reduced gain at low-frequencies and has a high-pass filter response. The resistors “seen” by the bypass capacitor include RE, re’, and the bias resistors.

The equivalent high-pass filter for C2 is:

Question:

How would an emitter swamping resistor affect the response?

fc would be lower due to increased R.

Electronic Devices

Internal Capacitances

The high-frequency response of an amplifier is determined by internal junction capacitances. These capacitances form low-pass filters with the external resistors.

Electronic Devices

Internal Capacitances

The high-frequency response of an amplifier is determined by internal junction capacitances. These capacitances form low-pass filters with the external resistors.

Sometimes a designer will add an external parallel capacitor to deliberately reduce the high frequency response.

Electronic Devices

Miller’s Theorem

Miller’s theorem states that, for inverting amplifiers, the capacitance between the input and output is equivalent to separate input and output capacitances to ground.

Electronic Devices

Miller’s Theorem

Miller’s theorem states that, for inverting amplifiers, the capacitance between the input and output is equivalent to separate input and output capacitances to ground.

Av is the absolute value of the gain. For the input capacitance, the gain has a large effect on the equivalent capacitance, which is an important consideration when using inverting amplifiers.

Electronic Devices

Miller’s Theorem

Notice that the effect of Miller’s theorem is an equivalent capacitance to ground, which shunts high frequencies to ground and reduces the gain as frequency is increased.

Electronic Devices

Miller’s Theorem

Notice that the effect of Miller’s theorem is an equivalent capacitance to ground, which shunts high frequencies to ground and reduces the gain as frequency is increased.

Example:

What is the input capacitance for a 2N3904 inverting amplifier with a gain of 25? Assume the values of Cbc = 4 pF and Cbe = 6 pF.

Electronic Devices

Miller’s Theorem

Notice that the effect of Miller’s theorem is an equivalent capacitance to ground, which shunts high frequencies to ground and reduces the gain as frequency is increased.

Solution:

Example:

What is the input capacitance for a 2N3904 inverting amplifier with a gain of 25? Assume the values of Cbc = 4 pF and Cbe = 6 pF.

Cin = Cbc(Av + 1) + Cbe

Cin = 4 pF(25 + 1) + 6 pF =

Electronic Devices

Miller’s Theorem

Notice that the effect of Miller’s theorem is an equivalent capacitance to ground, which shunts high frequencies to ground and reduces the gain as frequency is increased.

Solution:

Example:

What is the input capacitance for a 2N3904 inverting amplifier with a gain of 25? Assume the values of Cbc = 4 pF and Cbe = 6 pF.

Cin = Cbc(Av + 1) + Cbe

Cin = 4 pF(25 + 1) + 6 pF =

110 pF

Electronic Devices

The Decibel

Recall that the decibel was defined in Chapter 6 as a logarithmic measure the ratio of one power to another or one voltage to another. The decibel is used in electronics work in gain or attenuation measurements. Decibels can be expressed as a voltage ratio when the voltages are measured in the same impedance.

Electronic Devices

The Decibel

To express power gain in decibels, the formula is

Ap(dB) = 10 log Ap

Electronic Devices

The Decibel

To express power gain in decibels, the formula is

Ap(dB) = 10 log Ap

To express voltage gain in decibels, the formula is

Av(dB) = 20 log Av

Electronic Devices

The Decibel

Sometimes, 0 dB is assigned as a convenient reference level for comparison. Then, other power or voltage levels are shown with respect to 0 dB.

Electronic Devices

The Decibel

Sometimes, 0 dB is assigned as a convenient reference level for comparison. Then, other power or voltage levels are shown with respect to 0 dB.

Some useful decibel ratios to remember are:

Ratio	Power gain, Ap	Voltage gain, Av
0.1	-10 dB	-20 dB
0.5	-3 dB	-6 dB
1	0 dB	0 dB
2	3 dB	6 dB
10	10 dB	20 dB

Electronic Devices

The Decibel

Sometimes, 0 dB is assigned as a convenient reference level for comparison. Then, other power or voltage levels are shown with respect to 0 dB.

Some useful decibel ratios to remember are:

Ratio	Power gain, Ap	Voltage gain, Av
0.1	-10 dB	-20 dB
0.5	-3 dB	-6 dB
1	0 dB	0 dB
2	3 dB	6 dB
10	10 dB	20 dB

The -3 dB power gain corresponds to a power reduction of one-half. The frequency at which this occurs is referred to as the critical frequency.

Electronic Devices

Low-Frequency Response

In capacitively coupled amplifiers, the coupling and bypass capacitors affect the low frequency cutoff. These capacitors form a high-pass filter with circuit resistances. A typical BJT amplifier has three high-pass filters.

Electronic Devices

Low-Frequency Response

For example, the input coupling capacitor forms a high-pass filter with the input resistance of the amplifier:

Electronic Devices

Low-Frequency Response

The output RC circuit is composed of the series combination of the collector and load resistors with the output capacitor. The cutoff frequency due to the output circuit is

Electronic Devices

Low-Frequency Response

What is the lower cutoff frequency due to C1?

Example:

Assume re’ = 3.5 W and b = 200.

Electronic Devices

Low-Frequency Response

What is the lower cutoff frequency due to C1?

Solution:

Example:

Assume re’ = 3.5 W and b = 200.

RE1 is not bypassed, so it is added to re’. Then:

= 2.77 kW

Electronic Devices

Low-Frequency Response

What is the lower cutoff frequency due to C1?

Solution:

Example:

Assume re’ = 3.5 W and b = 200.

RE1 is not bypassed, so it is added to re’. Then:

= 2.77 kW

Electronic Devices

Low-Frequency Response

The bypass RC circuit response can be found by observing the charge/discharge paths.

For this circuit, there is one path through RE2 as shown in blue.

Electronic Devices

Low-Frequency Response

The bypass RC circuit response can be found by observing the charge/discharge paths.

For this circuit, there is one path through RE2 as shown in blue.

A second path goes through RE1, re’, and the parallel combination of bias and source resistances as shown in red (source resistance not shown).

Electronic Devices

Low-Frequency Response

The bypass RC circuit response can be found by observing the charge/discharge paths.

For this circuit, there is one path through RE2 as shown in blue.

A second path goes through RE1, re’, and the parallel combination of bias and source resistances as shown in red (source resistance not shown).

The total resistance of the paths can be found by:

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the bypass RC circuit?

(Assume Rs = 600 W and b = 200 and re’ = 2.6 W).

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the bypass RC circuit?

Solution:

(Assume Rs = 600 W and b = 200 and re’ = 2.6 W).

= 79.7 W

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the bypass RC circuit?

Solution:

(Assume Rs = 600 W and b = 200 and re’ = 2.6 W).

= 79.7 W

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the bypass RC circuit?

Solution:

(Assume Rs = 600 W and b = 200 and re’ = 2.6 W).

= 79.7 W

42.5 Hz

Electronic Devices

Low-Frequency Response

The input RC circuit for a FET is a basic high-pass filter consisting of the bias resistor (or resistors) and the input coupling capacitor. The FET gate circuit has such high resistance, it can be ignored.

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the input RC circuit?

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the input RC circuit?

Solution:

Electronic Devices

Low-Frequency Response

Example:

What is the critical frequency due to the input RC circuit?

Solution:

1.6 Hz

Electronic Devices

The Bode Plot

The Bode plot is a plot of decibel voltage gain verses frequency. The frequency axis is logarithmic; the decibel gain is plotted on a linear scale. The -3dB point is the critical frequency.

Electronic Devices

The Bode Plot

Multisim has a fictitious instrument called the Bode plotter. This is the previous BJT amplifier.

Electronic Devices

The Bode Plot

Multisim has a fictitious instrument called the Bode plotter. This is the previous BJT amplifier.

Connect the IN of the plotter to a constant level to the left of the Thevenin source.

Electronic Devices

The Bode Plot

Multisim has a fictitious instrument called the Bode plotter. This is the previous BJT amplifier.

The Bode plotter allows you to see the Bode plot directly. By selecting the proper scales, you can magnify the response. Move the cursor to the point where the total response is – 3dB from midband and read fc as shown here.

Set the cursor 3dB below the midband gain and read fc.

Electronic Devices

High-Frequency Response

The high frequency response of inverting amplifiers is primarily determined by the transistor’s internal capacitance and the Miller effect. The equivalent high-frequency ac circuit is shown for a voltage-divider biased CE amplifier with a fully bypassed emitter resistor.

Electronic Devices

High-Frequency Response

If there is an unbypassed emitter resistor, such as RE1 in the earlier example, it is shown in the emitter circuit and acts to increase re’ and thus reduce fc.

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