Download INPUT OFFSET CURRENT Ios

Analog Electronics
2130902
UNIT- OPERATIONAL AMPLIFIER
Branch:- Electrical (09)
PREPARED BY:PRITESH DESAI (020)
NIDHI GANATRA (026)
VIVEK GARALA (029)
INTRODUCTION
• Electronic systems usually process information in
either analog or digital form.
• In order to process the two different kinds of
signals, analog circuits and digital circuits have
been devised.
• Most circuits found in analog systems are linear
circuits in which one voltage (or current) is
meant to be linearly proportional to another.
• Linear active circuits are also known as
amplifiers.
• An amplifier can be modeled as a two-port device, that is,
a box with two pairs of terminals designated as input and
output, as shown in Figure
• Let the amplifier block be connected to a current source
at the input terminals, as shown in Figure and to a load
resistance RL at its output terminals. Find Vout.
PRACTICAL CHARACTERISTIC
OF OPERATIONAL
AMPLIFIERS
• The op-amp has high input impedance and low output
impedance. Therfore, it will not draw much current from
the connected external input voltage signal source. As a
result, the loading effect is avoided. Due to low output
resistance it delivers the maximum output. Based on these
characteristics, due to biasing voltages applied to the IC
and the current drawn by the IC from the DC-supplying
(bias) voltages, various parameters are defined.
 INPUT RESISTANCE Ri

OPEN-LOOP VOLTAGE GAIN A

OUTPUT RESISTANCE Ro
COMMON-MODE REJECTION RATIO (CMRR)
INPUT OFFSET VOLTAGE VOS
INPUT BIAS CURRENT IB
INPUT OFFSET CURRENT IOS
POWER-SUPPLY REJECTION RATIO(PSRR)
MAXIMUM DIFFERENTAIL INPUT VOLTAGE
MAXIMUM COMMON-MODE INPUT VOLTAGE
OUTPUT VOLTAGE SWING
INTERNAL FREQUENCY COMPENSATION
SLEW RATE
NOISE
STABILITY
FREQUENCY RESPONSE
INPUT RESISTANCE (Ri) :Input resistance is the open-loop incremental resistance
looking into the two input terminals, and is typically 2M.
Manufacturers sometimes quote the resistance between
inputs and ground.
OUTPUT RESISTANCE (Ro) :The open-loop output resistance is usually between 50
and 500 , with a typical value of 75 for the 741. Thus one can
see that Figure is the representation of an op amp as a circuit
element or block
OPEN-LOOP VOLTAGE GAIN A :The op amp amplifies the difference Vd between the
voltage on the non inverting (+) terminal and the inverting
(−) terminal; see Figure The term “open loop” implies that
there is no external feedback connection between the output
and either of the inputs. A is defined as the ratio of the
change in output voltage to the change in differential input
voltage, usually for a load resistance of no less than 2 k. A
typical value for A is 2 × 10 5
Common-Mode Rejection Ratio (CMRR):This parameter indicates the capability of the op-amp to
reject noise. The higher the value of CMRR, the better it is.
CMRR is defined as the ratio of the differential voltage gain,
Ad, to the common mode voltage gain, Acm. (cm = common
mode)
INPUT OFFSET VOLTAGE Vos :When both inputs are tied to ground, i.e., both
differential-mode and common-mode inputs are zero, the
output should be zero. In practice there will be mismatches in
amplifier components, and if there is a mismatch in an input
stage, the effect will be amplified, leading to a significant
output voltage. The input offset voltage VOS is the
differential input voltage required to make the output zero,
and is typically 1 mV. With some op amps
INPUT BIAS CURRENT IB :This is the average of the currents that flow into the
inverting and non-inverting input terminals of the op-amp.
For an ideal op-amp IB = 0 Amps
For a practical op-amp IB = 500 nA
For a precision op-amp, IB is typically 10 nA.
INPUT OFFSET CURRENT Ios:Though for an ideal op-amp, the input impedance is ∞, it
is not so practically. So the IC draws current from the source of
the voltage, however small it may be. The algebraic difference
between the currents into the inverting and non-inverting
terminals is referred to as input offset current Iio.
Iio = |IB1 − IB2|
OUTPUT VOLTAGE SWING:Ideally this is equal to the difference between the two
supply rail voltages, although in practice it is a few volts less.
INTERNAL FREQUENCY COMPENSATION:Some op amps, such as the 741, have internal RC
networks which are intentionally designed to reduce gain at
high frequency. The result (shown in Figure 5.2.4) is that the
open-loop gain begins to fall at a few hertz and then has a
characteristic falling at 20 dB/decade or 6 dB/octave (i.e.gain ∝
1/f ) until eventually the gain becomes unity (0 dB) at about 1
MHz
OUTPUT VOLTAGE SWING :Ideally this is equal to the difference between the two
supply rail voltages, although in practice it is a few volts less.
EXTERNALFREQUENCY COMENSATION:For applications requiring a more extended highfrequency response, there are op amps with no internal
compensation, and external frequency-compensation terminals
are provided on these op amps (for example, the 709) so that
the frequency response can be tailored to avoid instability
without the heavy degrading of frequency response due to
internal compensation.
.
SLEW RATE:
• This is defined as the maximum rate of change of output
voltage per unit time.
• The input capacitance of the op-amp circuit prevents it from
responding instantaneously to high frequency signals. If a
square wave input or pulse of high frequency is applied as
input, at the output the slope of the leading edge and trailing
edge is measured and the larger value is computed. This
gives SR.
NOISE:-
This refers to the small, rapidly varying, random
spurious signals generated by all electronic circuits. Noise
places a limit on the smallness of signals that can be used.
The subject of random signals and noise belongs to a branch
of electrical engineering known as communication theory.
STABILITY:-
The amplifier is said to be stable when it performs its
function reliably under all normal operating conditions. By
definition, a system is stable if its response to an excitation
that decays to zero with time also decays to zero. Instability
can come about in various ways, and it is a very common
difficulty, which must be prevented. In almost all cases, opamp circuits can be classified as feedback circuits. Feedback
of an improper kind can lead to instability or oscillation of an
op-amp circuit.
Frequency Response :As with all electronic circuits, op amps have limited
frequency response. Because of the negative
feedback of the circuit, the pass band of an op-amp circuit is
usually much larger than that of the op amp by itself. Typically
with a given op amp, increasing the bandwidth of an op-amp
circuit will decrease the voltage gain in the same proportion..
REFRENCE :INTRODUCTION TO
ELECTRICAL ENGINEERING
Mulukutla S. Sarma
Northeastern University
GUIDE BY :Prof. Tushar dave