Comparators
A comparator compares a signal voltage on one input of
an opamp with a known reference voltage on the other
input. We can say A comparator has two inputs one is
usually a constant reference voltage VR and other is a
time varying signal Vi and one output VO.
ZERO CROSSING DETECTOR
If in the above circuits , we take VRef to zero, the circuit
becomes that of zero crossing detector, If VR = 0, then slightest
input voltage (in mV) is enough to saturate the OPAMP and the
circuit acts as zero crossing detector as shown in waveforms
below
Limitations
If the input to a comparator contains noise, the output may
show error when Vin is near a trip point.
For instance, with a zero crossing, the output is low when vin
is positive and high when vin is negative. If the input contains a
noise voltage with a peak of 1mV or more, then the comparator
will detect the zero crossing produced by the noise.
SOLUTION
Schmitt Trigger Circuit
It is basically a comparator with
positive feedback.
REGENERATIVE COMPARATOR
Upper Threshold Voltage, Vupt = +Vsat (Rdiv1/[Rdiv1+Rdiv2])
When Vout = -Vsat, the voltage across Rdiv1 is called Lower Threshold Voltage (Vlpt).
The input voltage, Vin must be slightly more negaitive than Vlpt inorder to cause the
output Vo to switch from -Vsat to +Vsat. When the input voltage is less than Vlpt, the
output voltage Vout is at -Vsat.
Lower Threshold Voltage, Vlpt = -Vsat (Rdiv1/[Rdiv1+Rdiv2])
If the value of Vupt and Vlpt are higher than the input noise voltage, the positive
feedback will eliminate the false output transitions. With the help of positive feedback
and its regenerative behaviour, the output voltage will switch fast between the positive
and negative saturation voltages.
Hysteresis Characteristics
Since a comparator circuit with a positive feedback is used, a dead band condition
hysteresis can occur in the output. When the input of the comparator has a value higher
than Vupt, its output switches from +Vsat to -Vsat and reverts back to its original state,
+Vsat, when the input value goes below Vlpt. This is shown in the figure below. The
hysteresis voltage can be calculated as the difference between the upper and lower
threshold voltages.
Vhysteresis = Vupt – Vlpt
Subsituting the values of Vupt and Vlpt from the above equations:
Vhysteresis = +Vsat (Rdiv1/Rdiv1+Rdiv2) – {-Vsat (Rdiv1/Rdiv1+Rdiv2)}
Vhysteresis = (Rdiv1/Rdiv1+Rdiv2) {+Vsat – (-Vsat)}
Comparator Characteristics
• Speed of Operation- How quickly output switches
between two saturation levels, Higher Bandwidth
• Accuracy- The repeatable changes in the input that
causes output to switch. Voltage gain, CMRR, Input
Offset, Thermal Drift
• Compatibility of Output- TTL Compatible
Limitations of op-amp as Comparator
• Since output levels are ±Vsat, so they are not
compatible with TTL family. Thus we use
Voltage Limiters
Voltage Limiters
Barkhausen Criteria
Vd
Conditions which are required to be
satisfied to operate the circuit as an
oscillator are called as “Barkhausen
criterion” for sustained oscillations.
The Barkhausen criterion states that:
• The loop gain is equal to unity in absolute
Vd=Vf+Vin
magnitude, that is, | β A | = 1 and
Vo=AvVd, Vf=Bvo
• The phase shift around the loop is zero or an
𝑉𝑜 𝐴𝑣
integer multiple of 2π: ∠ β A = 2 π n, n ∈ 0, 1, =
2,…. 𝑉𝑖𝑛 1 − 𝐴𝑣𝐵
The product β A is called as the “loop gain”. Vin=0, V0≠0means AvB=1 =1ʟ0о or 360
о
Wien Bridge Oscillator
RC Oscillator
LC Oscillator
Crystal Osc.
Phase Shift Oscillator
Square Wave Generator
R2
Vut
Vlt
t1
𝑅1
𝑉𝑙𝑡 = −𝑉𝑠𝑎𝑡
𝑅1 + 𝑅2
𝑅1
𝑉𝑢𝑡 = +𝑉𝑠𝑎𝑡
𝑅1 + 𝑅2
Capacitor Charging/Discharging Equation
𝒕
−𝝉
𝑽𝒄 = 𝑽𝒇 + 𝑽𝒊 − 𝑽𝒇 𝒆
𝜏 = 𝑅𝐶, 𝑉𝑐 = 𝑉𝑢𝑡, 𝑡 = 𝑡1, 𝑉𝑓 = +𝑉𝑠𝑎𝑡, 𝑉𝑖 = 𝑉𝑙𝑡
𝑡1
−𝑅𝐶
𝑉𝑢𝑡 = +𝑉𝑠𝑎𝑡 + 𝑉𝑙𝑡 − 𝑉𝑠𝑎𝑡 𝑒
𝑉𝑙𝑡 − 𝑉𝑠𝑎𝑡
𝑡1 = RCln( )
𝑉𝑢𝑡 − 𝑉𝑠𝑎𝑡
2𝑅1 + 𝑅2
𝑇 = 2𝑡1 = 2𝑅𝐶𝑙𝑛
𝑅2
2𝑅1+𝑅2
Putting 𝑙𝑛 =1
𝑅2
R2=1.16R1, T=2RC
Triangular Wave Generator
Triangular Wave Generator
When comparator output is at +Vsat, the effective voltage at point P is given by
When effective voltage at P becomes equal to zero, we can write above equation
Similarly, when comparator output is at -Vsat ,we can write,
The peak to peak amplitude of the triangular wave can be given as
The time taken by the output to swing from – Vramp to + Vramp (or from + Vramp to – Vramp) is
equal to half the time period T/2. Refer Fig. This time can be calculated from the
integrator output equation as follows :
Substituting value of Vo(pp) we get,
Therefore, the frequency of oscillation can be given as
Design a triangular wave generator for fo=2 KHz, Vopp=7V, Supply Voltage= ±15V
Ans Assuming R2=10K, R3=40K(50K), C1=0.05uF(C<1uf), R1=10K
Sawtooth Wave Generator
• Triangular Wave– Rise time=Fall time
• Sawtooth Wave– Rise time is not equal to fall time
• The wiper R4 connected at A2 can be used to alter the rise time and fall
time as it adds a DC voltage to the integrator.
• When Wiper moves towards –VEE, the rise time becomes larger than
fall time
• When Wiper moves towards +Vcc, the fall time becomes larger than rise
time
Current to Voltage Converter
Voltage to Current Converter (Floating Load)
Current series negative feedback amplifier because the feedback voltage
across R depends on the output current iL and is in series with the input
difference voltage vd
Writing the voltage equation for the input loop.
vin = vd + vf
But vd is very small as A is very large, therefore,
vin = vf
vin = R iin
iin = v in / R.
and since current entering the op-amp is zero.
iL = iin = vin ./ R
Voltage to Current Converter(Grounded Load)
Since the collector and emitter currents are
equal to a close approximation and the input
impedance of OPAMP is very high, the load
current also flows through the feedback
resistor R. On account of this, there is still
current feedback, which means that the load
current is stabilized.
Since vd= 0
\ v2 = v1 = vin
\ iout = (vCC– vin ) / R
Thus the load current becomes nearly equal
to iout. There is a limit to the output current
that the circuit can supply.
The base current in the transistor equals
iout / bdc. Since the op-amp has to supply this
base current, iout / bdc must be less than
Iout (max) of the op-amp, typically 10 to 15mA.
