Introduction to Amplifiers

Outline

• Amplifier Properties

• BJT Amplifier Configurations

• Amplifier Classifications

• Decibels

Amplification

Amplificationthe process of increasing the power of an ac signal

BJT amplifier, JFET amplifier, OP-AMP amplifier

What is amplification?

Part 1.

Amplifier Properties

Amplifier Properties

• Three Fundamental Properties– Gain– Input impedance– Output impedance

outputinput

Zout

Zin A

General amplifier model

Amplifier Gain

• Gain– A multiplier that exists between the input and output

of a circuit.– For example, if the gain of an amplifier is 100, then

the output signal is 100 times as great as the input signal under normal operating conditions.

• Types of Gain:– Voltage gain, AV

– Current gain, Ai

– Power gain, Ap

Gain as a Ratio

• Gain– Ratio of an output value to its corresponding input

signal

Av = vout

vin

Where

Vout = the ac output voltage from the amplifier

Vin = the ac input voltage to the amplifier

Ai = iout

iin

Ap = Pout

Pin

The General Voltage Amplifier Model

outputinput

Zout

Zin Avvin

voltage amplifier model

Zout

Zin Avvin

RS

vS

RL

Voltage source

Amplifier Input Impedance (Zin)

• Input impedance (Zin)– The load that an amplifier places on its source.– “When an amplifier is connected to a signal source,

the source sees the amplifier as a load. The input impedance of the amplifier is the value of this load.”

vin = vs Zin

RS + Zin

Amplifier input circuit

Zout

Zin Avvin

RS

vSvin

1.5kΩ

100Ω

2 mV

Example.

Calculate vin.

vin = 2mV (1.5kΩ)/ 1.6kΩ

= 1.88 mV

Amplifier Output Impedance (Zout)

• Output impedance (Zout)– The source impedance that an amplifier presents to its load.– “When a load is connected to an amplifier, the amplifier acts as

the source for that load. As with any source, there is some measurable value of source impedance, in this casr, the output impedance of the amplifier.”

vL = vout RL

Zout + RL

Amplifier output circuit

Zout

Zin Avvin

RL

vL

1.2kΩ

300Ω

300 mV

Example.

Calculate vL.

vL = 300mV (1.2kΩ)/ 1.5kΩ

= 240 mV

Combined Effects of the Input and Output Circuits

• The combination of the input and output circuits can cause a fairly significant reduction in the effective voltage gain of an amplifier.

Av (eff) = vL

vS

Zout

Zin vout = Avvin

RL

1.2kΩ

250Ω

AV = 340

vin = 15mV (980Ω)/ 1kΩ = 14.7 mV

vout = 340(14.7) mV = 5 v

vL = 5v ( 1.2kΩ)/ 1.45kΩ = 4.14 v

RS

vS

20Ω

15 mV 980Ω

vS = 15mv vL = 4.14vAV(eff) = 4.14V/15 mV = 276

AV = 340 Av(eff) = 276

reduction of voltage

gain!

How do you reduce the effects of the input and output circuits on an amplifier voltage gain?

• 1. Increasing the value of Zin

• 2. Decreasing the value of Zout.

Av (eff) = vL

vS

Zout

Zin vout = Avvin

RL

1.2kΩ

20Ω

AV = 340

vin = 15mV (8kΩ)/ 8.02kΩ = 15 mV

vout = 340(15) mV = 5.1 v

vL = 5.1v ( 1.2kΩ)/ 1.22kΩ = 5 v

RS

vS

20Ω

15 mV 8 kΩ

vS = 15mv vL = 5vAV(eff) = 5 V/15 mV = 333

increased significantly!

Zin and Zout are affected by the choice of active components used as well as the type of biasing circuit and component values. Soon!!!

The Ideal Voltage Amplifier

• 1. Infinite gain (if needed).• 2. Infinite input impedance.• 3. Zero output impedance

Zout

Zin vout RL

1.2kΩ

0 ΩRS

vS

∞Ω

vin vL

no current in input circuit

vin = vS (ideal)

no voltage divider in output circuit

vL = vout (ideal)Av = AV(eff)

The Current Amplifier Model

• Current Amplifier – a circuit designed to provide a specific value of current gain.

ZoutZin Aiiin

ZoutZin Aiiin RLiS

RS

The Current Amplifier Model -Input Circuit-

• where:iin = amplifier input currentiS =the source currentRS|| Zin = the parallel combination of RS and Zin

iin < iS

ZoutZin Aiiin

iin = is RS || Zin

Zin

iS

Did we just use the current

divider formula?

The Current Amplifier Model -Output Circuit-• where:iL = amplifier load currentRL|| Zout = the parallel combination of RL and Zout

iout = Ai iiniL < iout

ZoutZin Aiiin

iL = iout RL || Zout

RL

RL

Combination of Input and Output circuit Effect reduced effective current gain

Ai(eff) = iL / iS solution: Decreasing the value of Zin

Increasing the value of Zout

The Ideal Current Amplifier

• Infinite gain (if needed).• Zero input impedance (Zin = 0Ω).• Infinite output impedance (Zout = ∞Ω)

iin = iS (for ideal current amplifier)

iL = iout (for ideal current amplifier)

Part 2.

BJT Amplifier Configurations

BJT Amplifier Configurations

• common- emitter amplifier

• common-collector amplifier

• common-base amplifier

Common-Emitter Amplifier• CE amplifier – is the most widely used BJT amplifier• The emitter terminal of the transistor is common to both input and

output circuits.• The emitter terminal of the transistor is normally returned to ac

ground (or ac common) provided by the “bypass capacitor” (CB).• The CE amplifier is unique it produces a 180° voltage phase shift

from its input to its output.

Common-Collector Amplifier• CC amplifier – is also known as emitter-follower• This circuit is most commonly used for its current

gain and impedance characteristics.

Common-Base Amplifier• CB amplifier – least often used BJT amplifier

configuration• The low input impedance and high output impedance of

the circuit are the exact opposites of the impedance characteristics of the ideal voltage amplifier.

Comparing the BJT Amplifier Configurations

AP = AV Ai

Common Emitter

Emitter Follower

Common Base

AvMidrange Less than 1 Midrange

AiMidrange Midrange Less than 1

APHigh Midrange Midrange

ZinMidrange High Low

ZoutMidrange Low High

Part 3.

Amplifier Classifications

Amplifier Classifications• Class A amplifier – an amplifier with a single transistor

that conducts during the entire input cycle.• Class B amplifier – an amplifier with two transistors that

each conduct for approximately half the input cycle.• Class C amplifier – an amplifier with one transistor that

conducts for less than 180° of the input cycle.• Class AB amplifier – an amplifier with two transistors that

each conduct for slightly 180° of the input cycle.

Amplifier Efficiency

Efficiency (η) – the percentage of the power drawn from the dc power supply than an amplifier actually delivers to its load.

η = (PL / Pdc ) x 100where: η = (eta) efficiency of the amplifier, in %

PL = ac load power

Pdc = dc input power

Distortion• One of the goals in amplification is to produce an output waveform

that has the same shape as the input waveform.• Distortion – any undesired change in the shape of a waveform• Two types of Distortion:

– Nonlinear distortion– Crossover distortion

Class A Amplifiers• Characteristics:

– An active device that conducts during the entire 360° of the input cycle.– An output that contains little or no distortion.– A maximum theoretical efficiency of 25%.

• Class A operation is achieved in a BJT amplifier by midpoint biasing the transistor.

• Because of their relatively poor efficiency ratings, class A amps are generally used as small-signal (low power) amplifiers.

Class B Amplifiers• Characteristics:

– Two transistors that are biased at cutoff (each conducts during one alternation of the ac input cycle).

– An output that contains little or no distortion.– A maximum theoretical efficiency of approximately 78.5%.

• The relatively high efficiency rating makes it very useful as a high-power amplifier.

Class AB Amplifiers• One variation of the class B amplifier.• Class B amplifier – an amplifier with two transistors that each

conduct for slightly more than 180° of the input cycle.• Also known as diode-biased amplifier.• This is used to prevent a specific type of distortion that can be

produced by a standard class B amplifier.

Class C Amplifiers• The BJT in the class C amp is biased deeply into cutoff. The ac input to the

amp causes the transistor to conduct for a brief time during the input cycle.• The output waveform is produced by the LC tank in the collector circuit.• Tuned amplifier – an amplifier designed to have a specific value of gain over

a specified range of frequencies.• Characteristics:

– A single transistor that conducts for less than 180° of the ac input cycle.– An output that may contain a significant amount of distortion.– A maximum theoretical efficiency rating of approximately 99%.

Circuit:

Conduction:

Maximum theoretical efficiency:

Distortion:

Part 4.

Decibels

Decibels• Decibel (dB) – a logarithmic unit used to express the

ratio of one value to another.• Writing numbers in dB form allows us to easily represent

very large gain values as relatively small numbers.

• dB Power Gain – the ratio of circuit output power to input power, equal to 10 times the common log of that ratio.

Ap(dB) = 10 log AP = 10 log (Pout / Pin)

Ap =log -1 (Ap(dB) / 10) Inverse log = antilog=(log-

1)

Positive versus negative dB values• Positive dB values represent a power gain, while

negative dB values represent a power loss.• Positive and negative decibels of equal magnitude

represent reciprocal gains and losses.

Say what?

Try this examples and compare.

1. Pin = 50 mW and Ap(dB) = 3 dB; Pout?

2. Ap(dB) = -3 dB and Pout = 50 mW, Pin?

The dBm Reference• This rating tells you that the maximum output power from

the amplifier is a certain value above 1 mW.• dBm values represent actual power levels, while dB

values represent power ratios.

Number 1. ….How can

this be?

Try this example.

1. An amplifier has a rating of Ap = 50 dB. Calculate the output power of the amplifier. Pout?

2. The output rating of an amplifier is given as 50 dBm. Calculate the output power for the circuit.

dB Voltage Gain

Av(dB) = 20 log Av = 20 log (vout / vin)

Ap =log -1 (Av(dB) / 20)

One Final Note on Decibels

1. Decibels are logarithmic representations of gain values.

2. Decibel power gain is found as 10 log AP.3. Decibel voltage gain is found as 20 lof AV.4. When AV changes by a given number of

decibels, AP changes by the same number of decibels.

5. You cannot use dB voltage and power gain values as multipliers. For example, if you want to determine vout, given vin and Av(dB), you must convert Av(dB) to standard numeric form before multiplying to find vout.