The differential a mplifier configuration is also called as long-tail pair as the two transistors share a
Notice that the operational amplifier has a positive power supply (+V CC) and a negative power supply (VEE). This arrangement enables the operational amplifier to produce either a positive or a negative output.
The two input terminals are labeled "inverting input" (-) and "non-inverting input" (+). The operational
amplifier can be used with three different input conditions (modes). With differential inputs (first mode),
both input terminals are used and two input signals which are 180 degrees out of phase with each other
are used. This produces an output signal that is in phase with the signal on the non-inverting input. If the
non-inverting input is grounded and a signal is applied to the inverting input (second mode), the output
signal will be 180 degrees out of phase with the input signal (and one-half the amplitude of the first mode
output). If the inverting input is grounded and a signal is applied to the non-inverting input (third mode),
the output signal will be in phase with the input signal (and one-half the amplitude of the first mode output)
Differential Amplifier Input Stage
common-emitter resistor. The current through this resistor is called the tail current. The base terminal of
transistor TR1 is the non-inverting input and base terminal of transistor TR2 is the inverting input. The
output is in-phase with the signal applied at non-inverting input and out-of-phase with the signal applied
at inverting input. If two different signals are applied to inverting and non-inverting inputs, the output is
given by Ad x (V1-V2)
8.2 Ideal Op-Amp
The ideal opamp model was derived to simplify circuit calculations. The ideal opamp model makes three
assumptions. These are as follows:
1. Input impedance, Zi = ∞
2. Output impedance, Zo = 0
3. Open-loop gain, Ad = ∞
From the above three assumptions, other assumptions can be derived. These include the following:
1. Since Zi = ∞, II = INI = 0.
2. Since Zo = 0, Vo = Ad x Vd.
3. Common mode gain = 0
4. Bandwidth = ∞
5. Slew Rate = ∞
6. Offset Drift = 0
Performance parameters
1. Bandwidth: Bandwidth of an opamp tells us about the range of frequencies it can amplify for a
given amplifier gain.
2. Slew rate: It is the rate of change output response to the rate of change in input. It is one of the
most important parameters of opamp. It gives us an idea as to how well the opamp output follows
a rapidly changing waveform at the input. It is defined as the rate of change of output voltage with
time. Slew rate limits the large signal bandwidth. Peak-to-peak output voltage swing for a
sinusoidal signal (Vp-p), slew rate and bandwidth are inter-related by the following equatio:
Bandwidth S lp ew´Vrpate - p
(
)
3. Open-loop gain: Open-loop gain is the ratio of single-ended output to the differential input.
4. Common Mode Rejection Ratio: Common Mode Rejection Ratio (CMRR) is a measure of the
ability of the opamp to suppress common mode signals. It is the ratio of the disered differential
gain (Ad) to the undesired common mode gain (Ac). æA
CMRR = 20logçç
d
ö
÷dB
è Ac ÷ø
5. Power Supply Rejection Ratio: Power Supply Rejection Ratio (PSRR) is defined as the ratio of
change in the power supply voltage to corresponding change in the output voltage. PSRR should
be zero for an Ideal opamp.
6. Input Impedance: Input Impedance (Zi) is the impedance looking into the input terminals of the
opamp and is mostly expressed in terms of resistance. Input impedance is the ratio of input voltage
to input current and is assumed to be infinite to prevent any current flowing from the source supply
into the amplifiers input circuitry (Iin =0).
7. Output Impedance: Output Impedance (Zo) is defined as the impedance between the output
terminal of the opamp and ground. The output impedance of the ideal operational amplifier is
assumed to be zero acting as a perfect internal voltage source with no internal resistance so that it
can supply as much current as necessary to the load. This internal resistance is effectively in series
with the load thereby reducing the output voltage available to the load.
8. Settling Time: Settling Time is a parameter specified in the case of high speed opamps of the
opamps with a high value of gain-bandwidth product
9. Offset Voltage (Vio): The amplifiers output will be zero when the voltage difference between the
inverting and the non-inverting inputs is zero, the same or when both inputs are grounded. Real
op-amps have some amount of output offset voltage.