Download File

Eulogio Amang Rodriguez
Institute of Science and Technology
Nagtahan, Sampaloc, Manila
Vacuum Tubes
Amplifiers
Introduction
The process of raising the strength of a
weak signal is known as amplification. The
device which accomplishes this job is called
an amplifier. All grid controlled Tubes (e.g.
triode, tetrode, pentode etc.) can act as
amplifiers. The weak signal is applied
between control grid and cathode and the
amplified output is obtained across the
load in the plate circuit.
The amplifying property of a grid voltage
causes a relatively large change in plate current
and hence greater output is Obtained across the
load in the plate circuit i.e. grid voltage is
amplified. It should be noted that an amplifier
amplifies only the electrical signal. This signal
to be amplified may be directly available or
converted into electrical By some suitable
device.
Triode as an Amplifier
• A triode is a grid controlled tube and can thus
act as an amplifier . The weak signal is
• introduced in the grid circuit and amplified
output is obtained across the load in the
• plate circuit. In order to achieve proper
amplification from a triode, it is necessary that
• The control grid remains negative w.r.t.
cathode at all times.
If grid is driven positive at any time by the input
ignal, the grid will draw current, Resulting in power
oss and distorted amplification. To ensure that grid is
lways negative w.r.t. cathode, one method can to
onnect a battery Ec in the grid circuit in addition to
he signal. This d.c. voltage is known as grid bias and is
lways of sufficient Magnitude and of such polarity so
s to maintain grid negative w.r.t. cathode at all
imes. Batteries are costly and require frequent
eplacement and therefore, they are Never used to
provide gird bias in vacuum tube amplifiers.
In practice, d.c. component Of plate current of triode
itself is used for obtaining grid bias voltage. The triode is then
said to be self-biased. Two commonly used methods of selfbiasing a triode are: cathode bias and grid leak bias.
Consider a triode amplifier having a.c. resistance of rp , load
resistance RL and amplification factor μ. then it can shown that
voltage gain Av of the circuit is given by;
Av = μ RL
Rp + RL
NOTE: It may be seen that the voltage gain of the circuit is less than u. it is because
some voltage is dropped across the internal resistance rp of the tube. If rp = 0, then
Av = μ.
Pentode as an Amplifier
As pentode is also a grid controlled tube, therefore, it
can act as an amplifier like a triode. However, the
amplification obtained in a pentode circuit is far superior to
that realised in a triode circuit. This is expected because a
pentode has very high value of u and little plate to Grid
feedback. The voltage gain of a pentode amplifier can be
calculated in a manner similar
To a triode amplifier and is given by ;
Av = μ RL
rp + R L
Where u, rp, and RL are the amplification factor, a.c.
plate resistance and load resistance respectively of the
pentode circuit. Generally rp>> RL so that the latter can be
neglected As compared to the former. With this assumption,
the voltage gain of a pentode amplifier
becomes:
Av = μ RL
rp
= 8 RL ( gm = μ / rp )
NOTE: This expression for voltage gain of a pentode amplifier is fairly
accurate and can be applied In practical circuits.
Multistage Amplifiers
The output from a single stage amplifier is usually insufficient to
drive an output Device, such as a loudspeaker. Additional amplification
over three to six stages is generally necessary. To accomplish this, output
of one stage is electrically coupled to the next stage through a suitable
coupling device. The resulting system is referred to as multistage
Amplifier.
The most commonly used methods of coupling are:
(i) R-C coupling
(ii) Choke-capacitance coupling
(iii) Transformer coupling
In each method of coupling, the object is to
transfer a.c. output of one stage to the input of the
next stage and to isolate the d.c. conditions of one
stage from the other. It may be noted that the use of
particular
coupling
depends
upon
service
requirements. For example, R-C coupling is best suited
for voltage amplification because of its distortionless
amplification. On the other hand, transformer
coupling is invariably Employed for power
amplification because it permits impedance matching.
Power Amplifiers
An amplifier which raises the power level of the
signal is called amplifier. The last stage of a multistage
amplifier is usually the power stage. Here,
concentrated effort is made to transfer maximum
power to the load. Obviously, transformer coupling
would be en appropriate device for such a situation.
Power amplifiers are generally classified on their
mode one operation i.e. plate current flow during
various parts of the signal.
i) A class A power amplifier is one in which grid bias is so
adjusted that plate current flows during all parts of the
signal. Class A operation gives distortion less output but has
the major limitation that plate efficiency is low (≡30%).
(ii) A class B power amplifier is one in which grid bias is so
adjusted that plate current flows during the positive half cycle
of the signal only. Class B operation gives distorted output but
has the advantage that plate efficiency is quiet high (≡50%).
Most of the power amplifiers employ two tubes in class B
operation; the circuit being known as push-pull circuit. One
tube amplifiers the positive half-cycle of the signal.
(iii) A class C power amplifier is one in which grid bias
is so adjusted that plate current flows for the less
than half-cycle of the input a.c. signal. The features of
class C operation are: higher distortion, high power
output and excellent plate efficiency (≡75%). Because
of their inherent distortion, the use of class C
amplifiers is only limited to oscillators and final stage
of radio transmitters where higher efficiency rather
than distortion is important.
Power Amplifiers
An amplifier which raises the power level
signal is called amplifier. The last stage of a mu
amplifier is usually the power stage. Here, concen
effort is made to transfer maximum power to th
Obviously, transformer coupling would be en appr
device for such a situation. Power amplifiers are ge
classified on their mode one operation i.e. plate C
flow during various parts of the signal.
(i) A class A power amplifier is one in which grid bias is so
adjusted that plate current flows during all parts of the
signal. Class A operation gives distortion less output but
has the major limitation that plate efficiency is low
(≡30%).
(ii) A class B power amplifier is one in which grid bias is
so adjusted that plate current flows during the positive
half cycle of the signal only. Class B operation gives
Distorted output but has the advantage that plate
efficiency is quiet high (≡50%).Most of the power
amplifiers employ two tubes in class B operation; the
circuit being known as push-pull circuit. One tube
amplifiers the positive half-cycle of the signal.
Because
of
their
inherent
distortion, the use of class C
amplifiers is only limited to
oscillators and final stage of radio
transmitters
where higher
efficiency rather than distortion is
important.
Because
of
their
inherent
distortion, the use of class C
amplifiers is only limited to
oscillators and final stage of radio
transmitters
where higher
efficiency rather than distortion is
important.
Important Terms in Power Amplifiers
The following terms are often used in the ana
power amplifiers:
(i)Plate power input(Pin).This refers to the
supplied to the plate circuit by the d.c.
supply and is equal to the product of the d.c
potential and plate
current i.e., Pin =Eb Ib
(ii) Plate power output (Po).This r
to the a.c. output from the tube
is equal to the product of the
component of plate voltage and
current i.e. Po=Ep Ip
(iii) Plate dissipation (P dis). This refers to
the power lost inside the tube in the
form of heat as a result of electron
bombardment of the plate structure.
Pdis=Pin-Po
(iv) Plate efficiency. This is the ratio of
a.c. output power and d.c. input
power i.e. Plate ῃ=(Po /Pin)x100%