A thyristor, commonly known as SCR (Silicon Controlled Rectifier) is basically a four – layer
pnpn device that has three electrodes ( a cathode, an anode and a control electrode called the
Current – voltage characteristics for an SCR are shown below:
When an SCR is reverse – biased (anode negative with respect to the cathode), the centre
junction is reverse – biased too and only a very low reverse current flows through the device.
If the voltage exceeds the reverse breakdown voltage, the reverse current increases rapidly.
During forward – bias operation (UAC is positive) the pnpn structure of the SCR is bistable
(OFF state – high impedance, ON state – low impedance. In the forward – blocking state
(OFF) a small forward current, called the leakage current, flows through the device. The
magnitude of this current is approximately the same as that of the reverse – blocking current.
At the forward breakover voltage the current increases rapidly and the SCR switches to the
ON state, with a very low (about 1V) forward ON – state voltage drop. In the ON state the
forward current is limited primarily by the load impedance. The SCR will remain in this state
until the current and then reverts back to the OFF state.
The breakover voltage can be varied by injection of a signal to the gate. As the gate current is
increased, the value of breakover voltage becomes less until the device goes into the ON state.
This enables an SCR to control a high power load with a very low – power signal.
Apart from the gate control, an SCR can also be triggered (turned ob.):
- by light incident onto the gate – cathode junction (phototyristor)
- when the anode – cathode voltage increases very rapidly
- when the temperature significantly increases.
The operation of the SCR can easily be explained using its equivalent two – transistor circuit.
When a current is applied to the gate, T2 turns on and its
collector drives the base of T1. T1 turns on and its collector
drives the base of T2.
The gate current is only required to trigger the device and can later cease to zero, because
upon triggering both transistors drive each other.
In order to turn off an SCR, the anode current must decrease below the holding current. In DC
circuits some additional components have to be used to ensure this. In AC circuits an SCR
turns off when the supply voltage (anode voltage) crosses zero towards negative values.
Some applications of thyristors
Thyristors are mainly used in power electronics. Power electronics deals with the application
of electronics devices and associated components to the conversion, control and conditioning
of electric power. The primary characteristics of electric power which are subject to control
include its basic form (AC or DC), its effective (RMS) voltage or current, frequency and
The control of electric power is frequently desired as a means for achieving control or
regulation of the speed of a motor, the temperature of an oven, the rate of an electrochemical
process, the intensity of lighting etc.
One of the simplest applications is a controlled rectifier, which can be obtained any rectifier
circuit by replacing the diodes with SCRs. The figure below shows a full – wave controlled
rectifier and the waveforms that illustrate its operation.
The average output voltage of this circuit is:
∫ U m sin ωtd ωt = Π (1 + cos α )
U LAV =
By varying the firing angle for both SCRs the average
rectified voltage can be changed from 2Um/Π
(uncontrolled rectifier) down to zero.
Note that the output waveform includes harmonics that may deteriorate the performance of
various electronic devices.
Many loads such as heaters, lamps etc. Do not require rectified supply voltage. In such cases
power can be directly controlled in AC circuits by use two SCRs connected in anti – parallel,
as shown below.
Inverters are used to convert DC into AC. This is accomplished through alternating
application of the source to the load, achieved through proper use of controllable switches.
Two simple structures of inverters are shown in the following figures.
push – pull inventer bridge inventer
In the push – pull inverter the DC supply source is periodically connected to each half of the
primary, producing a rectangular waveform across the load.
In the bridge inverter, which requires no power transformer, the SCRs are turned on in pairs,
T1T4 and T2T3. The capacitor is necessary to ensure the proper operation of the circuit, i.e. to
guarantee the commutation of SCRs.
With T1 and T4 in the ON state the capacitor is charged with the polarity shown in the figure.
When T2 and T3 are turned on, the voltage across the capacitor reverse biases both T1 and T4.
To produce a sinusoidal waveform of the output voltage, more sophisticated techniques are
Inverters are used, in particular, for control of electric motors, in uninterruptible power
supplies (UPS) and in many other applications.
Feedback is a commonly used method of modifying the parameters of electronic circuits. The
circuit shown in the figure illustrates a generalised view of feedback, with an amplifier with
gain k, and a feedback circuit which returns a fraction β of output to the input.
The returned signal Sf is summed with the input signal Sin to produce the input to the
amplifier Si. The input Si is often referred to as the error signal.
The signal at any node of the circuit can be either voltage or current.
Some basic definitions which apply are:
k f = is the open – loop gain, that is without feedback
S out is the feedback factor, that is a fraction of the returned signal
k = is the closed – loop gain, that is with feedback applied
For the general circuit the following relationships are valid:
Si =Sin +Sf
Sf = βSout
S out = k (S in + β S out )
The overall gain (closed – loop gain) is defined as:
S out k k
k = = or k =
1 − kβ 1+T
Where kβ is the loop gain, - kβ is often referred to as the loop transmission T, and 1- kβ is
known as the return difference (desensitivity).
Provided that 1- kβ>1 (kβ<0, either k or β negative) then kf < k and the feedback is said to be
If 1- kβ<1 then kf > k and the feedback is said to be positive.
The return difference 1- kβ can approach zero, and the closed – loop gain towards infinity.
Under these circumstances the circuit becomes unstable and oscillates.
Negative feedback offers a number of advantages and some of the more important are:
- improved gain stability
- reduced noise and distortion
- improved frequency response
- modified input and output impedance levels
These advantages are obtained at the expense of a reduction in the overall gain. However,
amplifiers are often designed to have a gain which is considerably higher than is required,
and negative feedback is used to reduce the gain to the required level with a subsequent
improvement in the overall performance of the circuit.
Positive feedback, on the contrary, has some disadvantages and is rarely used in electronic
circuits, mainly in oscillators.
Note that, in general, the phase relationship between the input and output of both the amplifier
and the feedback network varies with frequency. This means that it is possible for the
feedback to change from being negative to positive and the amplifier may oscillate. To
prevent this happening, special frequency compensation method must often be applied,
especially in wide – band amplifier circuits.
Types of feedback
Since both the output and the input can be characterised by either voltage or current, then four
possible types of feedback may be sampled and either a voltage or current may be returned to
the input. The four feedback configurations are shown below.
Notice different dimensions of the transfer parameter for the amplifier and the feedback
network in each case. In particular:
k= = ku is the voltage gain
u out is the transresistance
k= = Rm
k= = Gm is the transconductance
iout is the current gain
k= = ki
Effects of negative feedback
Let us start with an example, assuming that the open – loop voltage gain and the feedback are:
K=-10000 β = 0.1
k − 10000
Then kf = = = − 9 .990 ≈ 1
1 − k β 1 − ( − 10000 ) ⋅ 0 .1
Now let us assume that the open – loop voltage gain increases by 100% (e.g. due to the
The k f = = − 9 . 995 ≈ − 10
1 − ( − 20000 ) ⋅ 0 . 1
9 .995 − 9 .990
The increase in overall gain is ⋅ 100 % = 0 .05 %
Thanks to negative feedback a 100% variation bas been reduced to 0.05%. If the amplifier
were designed to have a gain of 10 without negative feedback then there could be large
variations of gain as a result of differences in the active components.
A common design practice is to arrange the loop gain to be much greater then unity.
k k 1
Then: kf = ≈ =−
1 − kβ − kβ β
According to this equation the gain of complete amplifier is independent of the open – loop
gain. The feedback factor β is usually determined by passive components, for example
resistors, which are much more stable than active devices.
Input and output resistance
Both the input and the output resistance are affected by the application of negative feedback.
With series feedback the voltage returned to the input opposes the input voltage and the input
current is less than it would be without feedback. This situation is shown in the following
The input resistance of the complete
amplifier is defined as
R inf =
ui − u f u i − kβu i u i
Rinf = = = (1 − kβ )
iin iin iin
where ui/iin is the input resistance of the amplifier without feedback.
Rinf = Rin (1 − kβ ) > Rin where kβ < 0
The input resistance with series feedback is increased, regardless of the output configuration
(whether the feedback signal is proportional to the output voltage or he output current).
For the shunt feedback:
uin u uin uin
Rinf = = in = =
iin ii − i f ii − kβii ii (1 − kβ )
1 − kβ
The input resistance with shunt feedback is reduced.
To determine the output resistance of an amplifier with feedback, we will use an equivalent
circuit of a voltage (for voltage sampling) or a current (for current sampling) amplifier.
The output resistance can be determined as the ratio of the open – circuit output voltage and
the short – circuit output current.
For the voltage sampling configuration we get:
uout u out
op.c op .c
iout iout sh.c
u out op . c = kS i = k ( S in + S f ) = k ( S in + β U out op . c
Uout op.c = kSi = k(Sin + Sf) = k(Sin + βUout op.c)
uout = Sin
iout sh.c. = = because sf = 0 (uout = 0)
kβ < 0 for negative feedback
The output resistance for voltage sampling with output resistance with either series or shunt
connection at the input is decreased.
It can be shown in a similar way that the output resistance for current sampling with either
series or parallel connection at the input is increased:
Rout f = Rout (1-kβ)
Reduction of noise and distortion
To study the effect of negative feedback on the noise, let us consider a two – stage amplifier
noise signal introduced into the signal path at different nodes.
With no feedback applied
Sout =k1k2 (Sin = n1 ) + k2n2 + n3
With negative feedback network connected:
k1 k 2 k2 1
S out = ( S in + n1 ) + n2 + n3
1 − k1 k 2 β 1 − k1 k 2 β 1 − k1 k 2 β
Comparing individual terms of both expressions it may seem that there is no improvement in
the signal to noise ratio. However, for given levels of noise entering the circuit and a required
value of the output signal, the contribution of individual noise signal into the output signal can
be significantly reduced.
If, for example n3=1V (e.g. ripple of the supply voltage), Sout=10V (e.g. maximum voltage
required to drive a loudspeaker), |k1|=|k2|=100 and β=0.1, then the ripple present in the output
signal becomes n3=1V with the feedback loop open.
n 3 = 1mV
1 − k1 k 2 β with the feedback loop closed
This reduction in noise level is achieved at a cost of signal amplification. With no feedback
the input signal should be 1mV (Sout/k1k2). With negative feedback applied the amplitude of
the input signal should be increased to 1V (kf=10).
Notice that the most efficient reduction is achieved for noise signal introduced into the circuit
near its output. Feedback cannot reduce noise which enters with the signal from the source.
The same conclusions can be drawn for the effect of negative feedback on non-linear
distortion. Non-linear distortion manifests as harmonics that appear in the output signal of an
amplifier. These harmonics can be considered as external distributing signal n3 that add to the
fundamental harmonic at the output node. Consequently:
1 − kβ
where h is the harmonics distortion coefficient for the open – loop amplifier.
Notice that negative feedback cannot reduce distortion that results from the transistor
operating point entering the cut – off or saturation region. In such cases the gain of the
amplifier becomes very low (=o) and the condition kβ>>1 is no longer valid.
Let us assume that the open – loop transfer function of an amplifier is described by the first –
k ( jω ) =
Where ko is the gain for low frequencies and ωH is the upper 3dB frequency.
Substituting this expression to the formula for the closed – loop gain we get:
k ωH k0 1 − k0 β kf0
k f ( jω ) = = = = =
1 − kβ k0 ω ω ω
1− β 1+ j − k0 β 1+ j 1+ j
ω ωH ω H (1 − k 0 β ) ω Hf
The closed – loop amplifier is also described by the first – order low – pass transfer function.
k f 0 ⋅ ω Hf = ⋅ ω H (1 − k 0 β ) = k 0ω H
1 − k0 β
In other words the closed – loop upped 3dB frequency is inversely proportional to the closed
– loop gain at low frequencies.
Taking the first – order transfer function of a high – order amplifier we can prove in a similar
way that the closed – loop lower 3dB frequency becomes
f Lf =
1 − kβ
In conclusion, at the expense of gain reduction by a factor of (1- kβ) negative feedback
reduces the lower 3dB frequency by the same factor. These effects are illustrated graphically
in the following figure.
Miller effect and bootstrap
Let us determine the impedance seen by a voltage that has an impedance connected between
its input and output as shown in the figure below.
Assuming that the amplifier is ideal (Rin → ∞, Rout = 0) we get:
U1 U 1 − kU 1 Z
Z in = I1 = Z =
I1 Z in 1− k
The impedance is scaled by a factor depending on he gain of the amplifier.
If a capacitor is connected as Z between the input and output of an inverting amplifier (k<0),
the capacitance seen by the source is:
Cin = C (1-k) = C(1+|k|)
This effective increase of C in known as the Miller effect. It is often used to linearize the
output waveform of ramp generators. On the other hand, it is often harmful because a large
increase in the input – to – output parasitic capacitance of an amplifier considerably decrease
If Z = R and k approaches 1, then Zin = Rin → ∞
This effect is know as bootstrapping. This technique is often used to dynamically increase the
resistance of a resistor. Another application deals with the linearization of the output
waveform in ramp generators as shown below.
U + E −U E
I= = = const
u (t ) =
C ∫ Idt =