6 Introduction To Amplifiers

Introduction to Amplifiers

Outline • • • •

Amplifier Properties BJT Amplifier Configurations Amplifier Classifications Decibels

Amplification Amplification the process of increasing the power of an ac signal BJT amplifier, JFET amplifier, OPAMP amplifier What is amplification?

Part 1. Amplifier Properties

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

input

Zin

A

General amplifier model

output

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

Ai =

iout iin

Where Vout = the ac output voltage from the amplifier Vin = the ac input voltage to the amplifier

Ap =

Pout Pin

The General Voltage Amplifier Model Zout

input

Zin

Avvin

output Voltage source

RS

vS

Zout

Zin

Avvin

voltage amplifier model

RL

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 100Ω

Zout

RS + Zin Example. Calculate vin . vin = 2mV (1.5kΩ)/ 1.6kΩ = 1.88 mV

vS 2 mV

vin

Zin 1.5kΩ Amplifier input circuit

Avvin

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

Zout + RL

300Ω RL

Example. Calculate vL.

Zin

Avvin 300 mV

vL = 300mV (1.2kΩ)/ 1.5kΩ = 240 mV

Amplifier output circuit

vL 1.2kΩ

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. RS 20Ω

vS 15 mV

Zout 250Ω

Zin

vout = Avvin

980Ω

RL 1.2kΩ

AV = 340 vin = 15mV (980Ω)/ 1kΩ = 14.7 mV

Av(eff)

vout = 340(14.7) mV = 5 v vL = 5v ( 1.2kΩ)/ 1.45kΩ = 4.14 v

vS = 15mv  vL = 4.14v

AV(eff)

= vL vS = 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 . RS 20Ω

vS 15 mV

Zout 20Ω

Zin

vout = Avvin

8 kΩ

RL 1.2kΩ

AV = 340 vin = 15mV (8kΩ)/ 8.02kΩ = 15 mV

Av(eff)

vout = 340(15) mV = 5.1 v vL = 5.1v ( 1.2kΩ)/ 1.22kΩ = 5 v

vS = 15mv  vL = 5v

AV(eff)

= vL

increased significantly!

vS = 5 V/15 mV = 333

Zin and Zout are affected by the choice of active components used as well as the type of biasing circuit and

The Ideal Voltage Amplifier • 1. Infinite gain (if needed). • 2. Infinite input impedance. • 3. Zero output impedance RS

vS

Zout 0 Ω

vin

Zin

vout

∞Ω

no current in input circuit vin = vS (ideal)

Av = AV(eff)

vL

RL 1.2kΩ

no voltage divider in output circuit vL = vout (ideal)

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

iS

RS

Zin

Aiiin

Zout

Zin

Aiiin

Zout

RL

The Current Amplifier Model -Input Circuitiin = is

RS || Zin Zin

iS

• where: iin = amplifier input current iS =the source current RS|| Zin = the parallel combination of RS and Zin iin < iS

Zin

Aiiin

Zout

Did we just use the current divider formula?

The Current Amplifier Model -Output CircuitiL = iout

RL || Zout RL

• where: iL = amplifier load current RL|| Zout = the parallel combination of RL and Zout iout = Ai iin iL < iout

Zin

Aiiin

Zout

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

Av

Midrange

Less than 1

Midrange

Ai

Midrange

Midrange

Less than 1

AP

High

Midrange

Midrange

Zin

Midrange

High

Low

Zout

Midrange

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 100

where:

η = (eta) efficiency of the amplifier, in % PL = ac load 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 highpower 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=(log1 )

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.

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 ?

Say what?

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. 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.

Number 1. ….How can this be?

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.