Download Experiment 1 - Electrical and Computer Engineering

ADDIS ABABA UNIVERSITY
INSTITUTION OF TECHNOLOGY
DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING
LABORATORY REPORT
Course number: Eceg: - 2206
Experiment Number: 01
Title:
Complimentary Symmetry Push-Pull Power
Amplifier
By: _
Group No.
:
Date of Experiment. 18-03-2012
Date of Submission. 01-04-2012
USED EQUIPMENT & COMPONENTS
No
Description
1
Variable DC Power source
2
Electronic or Voltmeter
3
Cathode Ray Oscilloscope
4
Conducting Wires
5
Decade Resistor
6
Transistor, 18k, 220, 4.7K, 1.8K,27K, 1k & 1.8K
7
Capacitors 10, 220, 470 µF & 27pF
8
Circuit Board
THE FREQUENCY RESPONSE OF A TRANSISTOR AMPLIFIER
I. INTRODUCTION
T
HE push–pull amplifier has shown excellent properties
at lower frequencies where they are easily implemented
by using n-p-n- and p-n-p-type bipolar junction transistor
(BJT) devices. However, at microwave and millimeter-wave
frequencies, FET devices are typically used with a hybrid or
balun to combine the power of the devices. While capable of
broad-band performance, good linearity and offering twice the
output power compared to a single-ended amplifier, the losses
in the output stage hybrid directly limits the practical efficiency
of this class of amplifier at microwave and millimeter-wave
frequencies [1]. For example, a combining loss of 0.5 dB will
decrease overall power-added efficiency (PAE) of two singleend
amplifiers operating at 60% to 53%. Various push–pull
amplifiers have been presented in the literature [2]–[4].
Active devices have been directly integrated with the antenna
platform to help solve many problems, including quasioptical
amplifiers and mixers, transceivers, and frequency
doublers [5]–[8]. Recently, the active integrated-antenna concept
has been applied to tuned high-efficiency power amplifiers
(PA’s) [9]–[11]. This approach has the advantage of placing
the PA directly at the antenna platform with minimal matching
and interconnects. This leads to lower output losses and
a compact transmitter front-end, critical for the stringent
requirements of today’s wireless systems. Increased efficiency
All amplifiers typically exhibit a band-pass frequency response as in Figure 1. The cut-off frequency on the low end is
usually determined by the coupling and bypass capacitors (if there are no such capacitors the low end extends all of
the way to DC). The high frequency limit is typically determined by internal capacitances in the transistor itself.
An amplifier is said to be linear if it preserves the details of the signal waveform, that
is to say,
(2.1)
where, Vi and Vo are the input and output signals respectively, and A is a constant gain
representing the amplifier gain. But if the relationship between Vi and Vo contains the higher
power of Vi, then the amplifier produces nonlinear distortion.
The amplifier’s efficiency is a measure of its ability to convert the dc power of the
supply into the signal power delivered to the load. The definition of the efficiency can be
represented in an equation form as
h DC power Supplied to output circuit
Signal power delivered to load
2.1 Amplifier Classification
Amplifiers are classified according to their circuit configurations and methods of
operation into different classes such as A, B, C, and F. These classes range from entirely
linear with low efficiency to entirely non-linear with high efficiency. The analysis presented
in this chapter assumes piecewise-linear operation of the active device. The majority of this
6
0 D i
( ) D m GS T i g V V
on
D
R
V
i 
D
information is available in Solid State Radio Engineering by Krauss, Bostain, and Raab
[1980].
The active device used in this research is the field effect transistor. The reason for
choosing this type of transistor is its superior performance in the microwave range
The characteristics of the FET can be described by:
cut-off region,
active region, (2.5)
saturation region.
The regions of operation are defined by:
cut-off region: VGS < VT ,
active region : VGS VT and iD < VD/Ron ,
saturation region: VGS VT and iD = VD/Ron .
The term “saturation” is used here to denote the region where further increase in gate voltage
produces no increase in drain current, that is to say, iD is independent of VGS.
2.2.1 Class A
The class-A amplifier has the highest linearity over the other classes. It operates in a
linear portion of its characteristic; it is equivalent to a current source. As shown in figures.2.1
and 2.2, the configurations of class-A, B, and C amplifiers can be either a push–pull or a single
ended tuned version. Figure.2.3 shows the load-line and current waveform for the class-A
amplifier. To achieve high linearity and gain, the amplifier’s base and drain dc voltage should
by chosen properly so that the amplifier operates in the linear region. The device, since it is on
7
(q) sin q D DD om V V V
dc DD dq
P V I
o om om DD dq
P V I V I
2
1
2
1
100 50%
2
1
100 
DD
om
dc
o
V
V
P
P
h
d dc o
P P P
(conducting) at all times, is constantly carrying current, which represents a continuous loss of
power in the device.
As shown in Fig.2.3, the maximum ac output voltage Vom is slightly less than VDD and
the maximum ac output current Iom is equal to Idq. In the inductor-less system, the output
voltage Vom will not be able to rise above the supply voltage, therefore, the swing will be
constrained to VDD/2 and not VDD. The drain voltage must have a dc component equal to that
of the supply voltage and a fundamental-frequency component equal to that of the output
voltage; hence
. (2.6)
The dc power is
, (2.7)
the maximum output power is
, (2.8)
and the efficiency is
. (2.9)
The difference between the dc power and output power is called power dissipation:
. (2.10)
2.2.2 Class B
The class-B amplifier operates ideally at zero quiescent current, so that the dc power
is small. Therefore, its efficiency is higher than that of the class-A amplifier. The price paid
for the enhancement in the efficiency is in the linearity of the device.
Figure 2.4 shows how the class-B amplifier operates. The output power for the singleended
class-B amplifier is
. (2.11)
the dc drain current is
, (2.12)
the dc power is
, (2.13)
and the maximum efficiency when Vom = VDD is
. (2.14)
o om o
P I V
2
1
p
om
dc
I
I 2
p
om DD
dc
IV
P

2
100 78.53%
4
100 
DD
om
dc
o
V
V
P
This article needs additional citations for verification. Please help improve this article by adding
citations to reliable sources. Unsourced material may be challenged and removed. (November 2010)
For other uses of "push–pull", see Push–pull (disambiguation).
A Class B push–pull output driver using PNP and NPN bipolar junction transistors configured as emitter
followers
A push–pull output is a type of electronic circuit that can drive either a positive or a negative current into a
load. Push–pull outputs are present in TTL and CMOS digital logic circuits and in some types of amplifiers,
and are usually realized as a complementary pair of transistors, one dissipating or sinking current from the
load to ground or a negative power supply, and the other supplying or sourcing current to the load from a
positive power supply.
Vacuum tubes (valves) are not available in complementary types (as are pnp/npn transistors), so the tube
push–pull amplifier has a pair of identical output tubes or groups of tubes with the control grids driven in
antiphase; these tubes drive current through the two halves of the primary winding of a center-tapped output
transformer in such a way that the signal currents add, while the distortion signals due to the non-linear
characteristic curves of the tubes subtract. These amplifiers were first designed long before the development
of solid-state electronic devices; they are still in use by both audiophiles and musicians who consider them to
sound better.
A vacuum tube amplifier often used a center-tapped output transformer to combine the outputs of tubes
connected in push-pull.
A Magnavox stereo tube push–pull amplifier, circa 1960, utilizes two 6BQ5 output tubes per channel
Contents
[hide]



1 Digital circuits
2 Analog circuits
o 2.1 Push-pull transistor output stages
 2.1.1 Transformer-output transistor power amplifiers
 2.1.2 Totem-pole push-pull output stages
 2.1.3 Symmetrical Push-pull
 2.1.4 Quasi-symmetrical push-pull
 2.1.5 Super-symmetric output stages
 2.1.6 Square-law push-pull
o 2.2 Push-pull tube (valve) output stages
 2.2.1 Ultra-linear push-pull
3 See also

4 External links
[edit] Digital circuits
The TTL output stage is a rather complicated push–pull circuit known as a ``totem-pole output (the
transistors, diode, and resistor in the right-most slice of this TTL logic gate circuit). It sinks currents better
than it sources current.
A digital use of a push–pull configuration is the output of TTL and related families. The upper transistor is
functioning as an active pull-up, in linear mode, while the lower transistor works digitally. For this reason
they aren't capable of supplying as much current as they can sink (typically 20 times less). Because of the way
these circuits are drawn schematically, with two transistors stacked vertically, normally with a protection
diode in between, they are called "totem pole" outputs.
In simpler digital circuits, especially in CMOS, each transistor is switched on only when its complement is
switched off.
A disadvantage of simple push–pull outputs is that two or more of them cannot be connected together,
because if one tried to pull while another tried to push, the transistors could be damaged. To avoid this
restriction, some push–pull outputs have a third state in which both transistors are switched off. In this state,
the output is said to be floating (or, to use a proprietary term, tri-stated).
The alternative to a push–pull output is a single switch that connects the load either to ground (called an open
collector or open drain output) or to the power supply (called an open-emitter or open-source output).
[edit] Analog circuits
A conventional amplifier stage which is not push–pull is sometimes called single-ended to distinguish it from
a push–pull circuit.
In analog push–pull power amplifiers the two output devices (transistors, tubes, FETs) or sets of devices
operate in antiphase (i.e. 180° apart). The two antiphase outputs are connected to the load in a way that causes
the signal outputs to be added, but distortion components due to non-linearity in the output devices to be
subtracted from each other; if the non-linearity of both output devices is similar, distortion is much reduced.
Symmetrical push–pull circuits must cancel even order harmonics, like f2, f4, f6 and therefore promote odd
order harmonics, like (f1), f3, f5 when driven into the nonlinear range.
A push–pull amplifier produces less distortion than a single-ended one. This allows a class A or AB push–pull
amplifier to have less distortion for the same power as the same devices used in single-ended configuration.
Class AB and class B dissipate less power for the same output as class A; distortion can be kept low by
negative feedback.
[edit] Push-pull transistor output stages
Categories include:
[edit] Transformer-output transistor power amplifiers
It is now very rare to use output transformers with transistor amplifiers, although such amplifiers offer the
best opportunity for matching output devices (with only PNP or only NPN devices required).
[edit] Totem-pole push-pull output stages
Two matched transistors of the same polarity (or, less often, Vacuum tubes) can be arranged to supply
opposite halves of each cycle without the need for an output transformer, although in doing so the driver
circuit often is asymmetric and one transistor will be used in a Common-emitter configuration while the other
is used as an Emitter follower. This arrangement is less used today than during the 1970s; it can be
implemented with few transistors (not so important today) but is relatively difficult to balance and so keep to
a low distortion (the highly non-linear TTL circuits such as the 7400 use this arrangement).
[edit] Symmetrical Push-pull
Each half of the output pair "mirror" the other, in that an NPN (or N-Channel FET) device in one half will be
matched by a PNP (or P-Channel FET) in the other. This type of arrangement tends to give lower distortion
than quasi-symmetric stages because even harmonics are cancelled more effectively with greater symmetry.
[edit] Quasi-symmetrical push-pull
In the past when good quality PNP complements for high power NPN silicon transistors were limited, a
workaround was to use identical NPN output devices, but fed from complementary PNP and NPN driver
circuits in such a way that the combination was close to being symmetrical (but never as good as having
symmetry throughout), and so distortion due to mismatched gain on each half of the cycle could be a
significant problem.
[edit] Super-symmetric output stages
Employing some duplication in the whole driver circuit, to allow symmetrical drive circuits can improve
matching further, although driver asymmetry is a small fraction of the distortion generating process. Using a
Bridge-tied load arrangement allows a much greater degree of matching between positive and negative halves,
compensating for the inevitable small differences between NPN and PNP devices.
[edit] Square-law push-pull
The output devices, usually MOSFETs, are configured so that their square-law transfer characteristics (that
generate second harmonic Distortion is used in a single-ended circuit) cancel distortion to a large extent. That
is, as the voltage across one transistor's gate-source voltage increases the remaining bias voltage to the
complementary device is reduced by that amount and the drain current change in the second device
approximately corrects for the non-linearity in the increase of the first.[1]
[edit] Push-pull tube (valve) output stages
See article: Valve audio amplifier – technical#The push-pull power amplifier.
These usually involve an output transformer to drop the output impedance to levels suitable for loudspeakers,
although Output-transformerless (OTL) tube stages exist (such as for headphones, for 100 Volt line Public
address sound systems, or for rare high-impedance loudspeakers).
[edit] Ultra-linear push-pull
Pentodes and Tetrodes can have their screen grid fed from a percentage of the primary voltage on the output
transformer, giving efficiency and distortion that is a good compromise between triode (or Triode-strapped)
power amplifiers circuits and conventional pentode or tetrode output circuits where the screen is fed from a
relatively constant voltage source. See article: Ultra-Linear.
1.
^ Ian Hegglun. "Practical Square-law Class-A Amplifier Design". Linear Audio - Volume 1.
Low frequency response
If an amplifier does not have coupling or bypass capacitors, then in general the low frequency response goes all of the
way down to DC. However, as we discussed in class, it is desirable to have these capacitors in the circuit to isolate the
amplifiers DC bias point from the outside world.
High Frequency Response
the high frequency response of a discrete transistor amp is determined by the internal capacitances of the transistor
itself
Transitional Frequency is the frequency at which the short circuit common emitter current gain becomes unity.
Therefore, Frequency response of a circuit means variation of the phase, gain and other parameters
caused as a result of changes in frequency. Note that all of this is done while making sure that the
amplitude of the voltage is kept constant at a certain value.
CALCULATIONS
Voltage Gain is defined as the ratio of the output
voltage to input (Vo/Vi)
Cbe 
I CQ
1

2f T hib 52f T x10 3
1
f High 
f High 
2 ( Rin Ri )(Cbe  CoB (1  Av ))
1


2CoB ( RB  ( RE RL ) Ri
f High 
1
2Cbe (hib RE Ri )
f Low _ C 2 
f Low _ C1 
1
2 2
1
21


Concerning the phase shift properties, we utilize a
special kind of mode of the CRO called the x-y
A Lissajous pattern figure can be used to
determine the phase difference angle (θ) between
the input signal (es) and out put amplified signal
(e0).
1
2C 2 ( Ri  Rin )
1
2C1 ( Ro  RL )
PROCEDURE
1. The circuit was setup according to circuit figure, and VCC was set to 9V, while checking VCE to be around 3.5 V.
and IC 2mA.
2. The signal generator was connected at the input terminal generating a 1Khz, 20 mVpp. And the value of the
output voltage e0 was measured. Again the process was repeated for es = 30 mVpp and the Amid was calculated.
3. Considering the frequency range of 10 Hz to 1 MHz, the Gain vs Frequency graph was plotted by taking all the
necessary measurements, this was done while keeping es constant at 20mVpp.The gain was determined for
each frequency with a capacitor ‘c’ and without it
4. The capacitor ‘c’ was removed from the circuit and both the input and output terminals of the network were
connected to the two channels of the CRO, and then using the phase difference comparison mode (Lissajous
Figure), the phase angle was determined for the same frequency ranges as was used in procedure 5.3.
9V
1.8 KΩ
4.7µF
120 KΩ
10µF
27 KΩ
3.9 KΩ
56 KΩ
1 KΩ
es
220µF
Circuit Layout Diagram
RESULTS
1. For a frequency of 10 Khz, with es =20mVpp and e0 = 0.6Vpp
The corresponding Gain will be: Gain = 0.6Vpp/20mVpp = 300
2. For a frequency of 20 Khz, with es =20mVpp and e0 = 0.8Vpp
The corresponding Gain will be: Gain = 0.8Vpp/20mVpp = 400
3. For a frequency of 150 Khz, with es =20mVpp and e0 = 1.2Vpp
The corresponding Gain will be: Gain = 0.6Vpp/20mVpp = 600
Input AC
10 20 50
100
200 500 1K
2K
5k
20K
50K
100K 200K
500K
1MHz
0.6
0.6 0.56 0.56 0.48
0.4
0.32
0.06
0.02
4
2
Frequency
e0
Td in ms
0.5 0.52 0.6
3.2
8
0.24 0.1
24µs 9.4µs 3µs
0.12
2.8µs 0.8µs 0.2µs
CONCLUSION
To briefly restate the main concepts we grasped upon completion of the laboratory session, we were able to understand many
things, each will be discussed under its own title.
Frequency response of a circuit means variation of the phase, gain and other parameters caused as a result of changes
in frequency. Note that all of this is done while making sure that the amplitude of the voltage is kept constant at a
certain value. FT: or Transitional Frequency is the frequency at which the short circuit common emitter current gain
becomes unity.
The Transistor is composed of three major parts in the semiconductor matrix, these are: The Emitter, the Base and the
Collector. All of the input and output characteristics of the network can be measured by connecting the appropriate
measuring meter as shown by the circuit diagram figure (A).
Although it wasn’t part of the procedure, our instructors have demonstrated to us that a special kind of Oscilloscope
can be used to depict the VCE versus ICE Graph.
For the proper functioning of the Transistor, certain values of current and voltage (also inherently, power) should not
be surpassed. These values are called: Current Rating, Voltage Rating and Power rating respectively. And even though
the values weren’t imprinted on the transistor we worked on, the laboratory manual we used instructed for us not to
surpass the following value, so, this value can be considered as the Current Rating value,: IB = 250µA.
Transistors can be used to amplify voltages, as observed from the laboratory session, the gain of 40, implies that the
input voltage has been magnified 40 times, and hence the transistor has been used as an Amplifier.