Op Amps Flashcards
Rectification
Conversion of alternating current to direct current
Half Wave Rectifier Circuit
AC voltage source connected to a forward biased diode connected to a load resistor connected to a grounder
Diode is forward biased during each positive half cycle causing current flow in the circuit
This current results in voltage across the resistor
Output voltage is pulsating DC which has a significant AC component and DC component which is the average value of V out
Smoothened Half Wave Rectifier
In series: AC source to a diode to a capacitor to a grounder
In parallel: Loaded resistance across the capacitor
During positive half cycle as Vs increases the diode conducts allowing the capacitor to charge to peak voltage of the sinusoid
After the peak, voltage starts decreasing make the anode of the diode at lesser voltage than cathode as the capacitor C holds the voltage at cathode at the peak voltage
This reverse biases the diode and the capacitor C starts discharging through the resistor until the voltage across it becomes less than Voltage which is rising sinusoid
Diode then gets forward biased and starts conducting until the peak is reached and this cycle continues
Ripple Factor
A measure of effectiveness of a rectifier circuit - defined as the ratio of RMS value of the AC component in the output waveform to the DC component in the output waveform
r = Vrms / Vdc
V rms = sqrt( (Vrms squared - Vdc squared) )
I rms = sqrt( (Irms squared - Idc squared) )
Irms = Im / 2
Idc = Im / pi
Ripple Factor - Half Wave Rectifier
Ripple factor can be significantly reduce using a filter capacitor
ripple factor for a half wave rectifier with filter capacitor is given by:
r = 1 / 2sqrt(3) x f x R(L) x C
r = (Vrpp / 2sqrt(3) ) / (Vp - 0.5Vrpp)
Bridge Rectifier
During the positive cycle of Vs, point A is positive with respect to B causing diode D1and D2 to forward bias and D3 and D4 to get reversed biased.
This results in the current to flow from A, through D1, R, D2, B to A.
During negative cycle, the polarities change.
Now point B is positive with respect to A causing diodes D3 and D4 to conduct and D1and D2to reverse bias.
The resulting current then flows from B through D4, R, D3, A to B.
In both the cycles, the current through the load resistor R flows in the same direction ensuring the pulsating DC across R in both the half cycles of the input voltage.
The output voltage can be smoothened by connecting a suitable capacitor across the load resistor
Ripple factor Half Wave Rectifier
r = Vrms / Vdc
Vdc = 2Vm / pi
r = 1 / (4rt3 x f x R(Load Resistor) x C)
Connections on an op-amp
Inverting V- and non inverting V+
One output
Power connections (V s+ and V s-) - active device
Voltage gain when a signal is amplified
Gain = A = ๐๐๐๐๐๐ ๐๐๐๐๐๐๐ / ๐๐๐๐๐๐๐ ๐๐๐๐๐ = ๐ฝ๐๐๐ /๐ฝin
The gain may be either:
open-loop (AOL) Here, none of the output from the amplifier is fed back into the input
(b) closed-loop gain (ACL ) Where, some of the output from the amplifier is fed (looped back) into the input
Ideal Op Amps
Infinite open loop gain
Infinite input resistance (between the V+ and Vterminals)
0 (zero) output resistance
A maximum output voltage of V+
A minimum input voltage of V-
Real Op Amps
an open loop gain of at least 10^5
An input resistance of 10^7 - 10^12
Output resistance of anywhere between 1-several kilo ohms (typical value 70 ohms)
A maximum output voltage of between 1-2 V (less than Vs+)
A minimum input voltage of 1-2 V (more than Vs-)
Non inverting input
V+ is the โnon-invertingโ input. The output will change in the same direction as the input applied to this terminal
Inverting inputs
V- is the โinvertingโ input. The output will change in the opposite direction as the input applied to this terminal
Transfer function
Vout = (A Open Loop)(V+ - V-)
When AOL is very high (>10^5) - (V+ - V- ) can be seen to plateau off.
The output has been driven into saturation (when supply voltages = Vs+ and Vs-)
Vout will not increase any further
A unity gain buffer amplifier
A unity gain buffer ( unity-gain amplifier) is a op-amp circuit which has a voltage gain of 1. This means that the op amp does not provide any amplification to the signal.
This is a special case of the non-inverting amplifier.
It has a high input resistance and a low output resistance and is therefore useful as
a buffer amplifier
Used as a โline driverโ that transforms a high input impedance (resistance) to a low
output impedance. Can provide substantial current gain. Negative feedback
