POWER AMPLIFIER(Additional Lecture Notes)

EKT 104

DC and AC Equivalent Circuits

RC

R1

+VCC

RE

R2

RL

vin

RC

R1

+VCC

IC

IE

RE

R2

R1//R2

rCvce

rC = RC//RL

vin

Bias Circuit DC equivalent circuit

AC equivalent circuit

Load Lines

• Every amplifier has two loads: a DC load and an AC load.

• DC load line– all possible DC combinations of IC and VCE.

• AC load line – all possible AC combinations of iC and vCE.

• AC load line involves Rc||RL in its determination, hence it is steeper than the DC line

*Q point is shared by both DC and AC load lines. However, it is determined on the DC load line.

DC Load Line

IC(mA)

VCE

VCE(off) = VCC

IC(sat) = VCC/(RC+RE)

DC Load Line

The straight line is known as the DC load line

The collector current IC and the collector-emitter voltage VCE must always lie on the load line, depends ONLY on the VCC, RC and RE (i.e. The dc load line is a graph that represents all the possible combinations of IC and VCE for a given amplifier. For every possible value of IC, and amplifier will have a corresponding value of VCE.)

What is IC(sat) and VCE(off) ?

Purpose of the DC biasing circuit

• To turn the device “ON” • To place it in operation in the region of its

characteristic where the device operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE

DC Biasing Circuits

RC

RB

+VCC

ic

vceib

v in

v out

The ac operation of an amplifier depends on the initial dc values of IB, IC, and VCE.

By varying IB around an initial dc

value, IC and VCE are made to

vary around their initial dc values.

DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device.

Q-Point (Static Operation Point)

• When a transistor does not have an ac input, it will have specific dc values of IC and VCE.

• These values correspond to a specific point on the dc load line. This point is called the Q-point.

• A quiescent amplifier is one that has no ac signal applied and therefore has constant dc values of IC and VCE.

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Q-Point (Static Operation Point)• The intersection of the dc bias

value of IB with the dc load line determines the Q-point.

• It is desirable to have the Q-point centered on the load line. Why?

• When a circuit is designed to have a centered Q-point, the amplifier is said to be midpoint biased.

• Midpoint biasing allows optimum ac operation of the amplifier.

DC Biasing + AC signal• When an ac signal is applied to the base of

the transistor, IC and VCE will both vary around their Q-point values.

• When the Q-point is centered, IC and VCE can both make the maximum possible transitions above and below their initial dc values.

• When the Q-point is above the center on the load line, the input signal may cause the transistor to saturate. When this happens, a part of the output signal will be clipped off.

• When the Q-point is below midpoint on the load line, the input signal may cause the transistor to cutoff. This can also cause a portion of the output signal to be clipped.

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DC Biasing + AC signal

AC Load Line

What does the ac load line tell you?• The ac load line tells the maximum

possible output voltage swing for a given amplifier---- the maximum possible peak-to-peak output voltage (Vpp ) from a given amplifier.

• The maximum undistorted Vpp is referred to as the compliance of the amplifier.

• The smaller of the two swings limits the maximum undistorted collector current for a given amplifier.

AC Load Line• The ac load line of a given

amplifier will not follow the plot of the dc load line.

• This is due to the dc load of an amplifier is different from the ac load.

IC(mA)

VCE

VCE(off) = VCC

IC(sat) = VCC/(RC+RE)

DC Load Line

IC

VCE

IC(sat) = ICQ + (VCEQ/rC)

VCE(off) = VCEQ + ICQrC

ac load line

IC

VCE

Q - point

ac load line

dc load line

* (AC Saturation Current Ic(sat) , AC Cutoff Voltage VCE(off) )

AC Load Line: The Upper Swing

• The current can swing from the Q point value to ic(sat).

- In this example, from 1.1mA to 3.52 mA.

• vCE can change from the Q point value to zero.

- In this example, from 4.94 V to 0 V.

AC Load Line: The Lower Swing• The current can swing from the Q

point value to zero. - In this case, from 1.1mA to 0 mA.

• vCE can change from the Q point value to vCE(off).

- In this example, from 4.94 V to 7.18 V.

• The voltage swing is determined by ICQrC.

- In this case, the maximum value of ICQrC is (1.1 mA)(2.04k Ohm)=2.24 V.

* This means that as the collector current swings between 1.1 mA and zero, the value of vCE will vary from 4.94 V to 7.18 V

Calculating Compliance• The smallest of the two (upper

swing and lower swing) determines the maximum possible peak voltage that can pass undistorted throughout our amplifier.

• In the example, the maximum upper swing peak voltage is 4.94 Vpk and the minimum lower swing peak voltage is 2.24 Vpk .

• Two times of the maximum possible peak voltage will give the maximum peak-to-peak transition value of the output voltage.

Calculating Compliance• The maximum peak-to-peak swing is given

by: PP=2VCEQ OR PP=2ICQrC

• In the example, max peak-to-peak value is: 2(4.94 Vpk)=9.88 Vp-p OR

2(2.24 Vpk)= 4.48 Vp-p• Since 4.48 Vp-p is the smaller of the two---it

is the compliance of the amplifier.• The Q point is below the mid-point of AC

load line. Cutoff clipping happens when the output voltage exceeds the compliance of the amplifier as shown in Waveform (A).

• Waveform (B) is limited to the compliance of the circuit (4.48 Vp-p)---it is not clipped and is undistorted.

Cutoff Clipping

• The Q point is below the midpoint and the output voltage is clipped off at the value of Vce(off).

Cutoff Clipping

Saturation Clipping

• The Q point is above the midpoint and the output voltage is clipped off when the amplifier hits saturation. At this point, the value of Vce Is virtually 0.

Vce(off)

Saturation Clipping

Efficiency

• The ideal power amplifier would deliver 100% of the power it draws from the power supply to the load.

• We know that this is not true and that components in the amplifier will all dissipate some power that is being drawn from the supply.

Where

PL = average ac power to the loadPS = average power supplied by the source (VCC)

AMPLIFIER DC POWER

• The DC source supplies direct current to the voltage divider and to the collector circuit.

• The total supply current is the divider current plus the quiescent collector current. ICC=ICQ+I1

• The total dc power that the amplifier draws from the power supply is found as: Ps=VCCICC

AC Load Power

• The ac load power is the power that is transferred to the load.

• The ac load power can be calculated as follows

Exercise 1• For the transistor in the common-emitter

circuit in Figure Ex. 1, the parameters are: β=80, PD,max =10W, VCE(sus) =30V, and IC,max

=1.2A.• (a) Design the values of RL and RB for VCC =

30 V. What is maximum power dissipated in the transistor?

• (b) Using the value of RL in part (a), find IC,max and VCC if PD,max = 5 W.

• (c) Calculate the maximum undistorted ac power that can be delivered to RL in parts (a) and (b) for the assumption that iC ≥0 and 0≤vCE ≤VCC.

Figure Ex.1

Exercise 1: Solution (a)

Exercise 1: Solution (b)

Exercise 1: Solution (c)