Circuit Symbols of OP-AMP
Input offset voltage
Input offset current
Input BIAS current
Large signal voltage gain
Output voltage swing
Differential input resistance Ri
Input capacitance Ci
Common mode rejection ratio
Supply voltage rejection ratio
Slew rate
Gain bandwidth product
What is an Ideal Op Amp?
An ideal op amp is an op amp that has perfect conditions to allow it to
function as an op amp with 100% efficiency.
An ideal op amp will display the following characeristics, of which are
all explained in detail below. Ideal op amps will have infinite voltage
gain, infinitely high impedance, zero output impedance, its gain is
independent of input frequency, it has zero voltage offset, its output can
swing positive or negative to the same voltages as the supply rails, and
its output swings instantly to the correct value.
In real life, as with all ideal components, an ideal op amp does not exist.
However, if we can get an op amp to display as close as possible the
characteristics of an ideal op amp as closely, we can make a more
efficient op amp that has better output in real-world conditions.
Now we will go over all of these characteristics of an ideal op amp
mentioned above, so that you can know what each is and so the
difference between an ideal case and a real case. Below is a table that
charts each of the major characteristics of an op amp and how they differ
in ideal and real op amps.
Ideal Op Amp Characteristics
Characteristics
Infinite
Gain
Ideal Op Amp
Real Op Amp
Voltage An ideal op amp will have A real op amp can
infinite voltage gain. Op only produce a finite
amps are devices that many gain.
times are used to function as
amplifiers. A voltage is input
into the op amp and as
output,
it
produces
the
voltage amplified. An ideal
op amp will produce megagain, practically, it will be
able to produce infinite gain.
It will amplify the signal
infinite times over so that we
can have as much gain as
we'd ever need.
An ideal op amp will have
infintely
high
input
impedance. This will ensure
that the op amp causes no
loading in the circuit. The
lower the input impedance,
the more current that an op
Infinitely
high
input impedance
amp will draw. The higher
the impedance, the lower the
current that an op will draw.
We
want
high
input
impedance so that the op
amp
doesn't
disturb
the
original circuit by pulling
A real op amp has
finite
input
impedance.
Even
though many types of
op
amps,
such
MOSFETs,
as
have
extremely high input
impedance,
in
the
order of teraohms, it is
still finite.
current from it. To do this,
we need infinitely high input
impedance.
Zero
Output An ideal op amp will have A real op amp will
Impedance
zero
output
impedance. always
When an op amp produces output
have
some
impedance,
its output signal, we want though it is low. A
the op amp to have zero typical value can be
voltage so that the maximum
voltage will be transferred to
the output load. Voltage is
divided
in
circuit
according to the amount of
impedance
present
in
circuit. Voltage drops across
a
component
of
higher 75.
impedance. In order for the
voltage to drop across the
output load, that load must
be of greater impedance than
the output of the op amp.
This is why, ideally, we want
the output impedance of the
op amp to be zero
Gain
Independent
Frequency
In an ideal op amp, the gain In real op amps, the
of that the op amp produces gain that is produced
will
be
independent
of is only for a certain
frequency. This means that bandwidth
regardless of the frequency frequencies.
of
Outside
of the input signal going into of this bandwidth, the
the op amp, the gain that is gain that the op amp
produced will be constant
and
good
across
all produces will decline.
frequencies.
In an ideal op amp, if no A real op amp will
voltage is applied to the have slight offset even
inverting and noninverting if the voltage applied
Zero
Input input pins, the op amp will to the pins are the
Voltage Offset
output a voltage of 0, since same. To correct this
there is no difference at all offset, voltage must be
of the voltage applied to the applied to the offset
2 input pins.
pin.
In an ideal op amp, the ac
voltage which is fed into the
op amp to be amplified will
Positive
and swing all the way up for the
Negative Voltage DC positive supply rail and
Swings to Supply all the way down for the DC
Rails
negative supply rail, making
100% efficient use of the
In real op amps, the
amplified signal will
not fully reach the DC
supply rails. They will
fall short of it.
DC voltage supplied to an
op amp.
Output
swings In an ideal op amp, the In real op amps, the
instantly to the output will swing instantly amplified signal will
to the amplified voltage take time to reach the
value. There will be no time fully amplified voltage
correct value
delay between the time the value.
voltage is input into the op determined
This
by
is
the
amp till the time it is output. slew rate of the op
It will all be instantaneous.
amp.
Again, ideal op amps can't exist because op amps, as all electronic
components, will have some internal resistance, which won't allow
maximum efficiency. However, if we can get real op amps as closely as
possible to ideal conditions, we will have very efficient, useful op amps
OP AMP Parameters
Operational Amplifiers An Introduction:
Operational Amplifiers (in short op amps) are made from discrete
components.
The opamp is a direct coupled high gain(negative feedback)
amplifier.
It can amplify signals having frequency ranging from 0Hz to
1MHz.
It is used to perform a wide variety of linear functions and some
non linear functions too.
Thus it is also called as basic linear integrated circuit.
This device is named as Operational amplifier because it was originally
designed
to
perform
mathematical
operations
like addition,
subtraction, differentiation, integration and multiplication etc in an
analog computer.
The main properties of an op amp are:
1.
A very high open loop voltage gain AO = 105 for d.c. and lowfrequency a.c., which decreases with frequency increase
2.
A very high input impedance (Ri = 106 to 1012) so that the input
voltage is passed on to the op amp with little loss
3.
A very low output impedance(RO around 100) , such that
efficiently the output voltage(VO) is transferred to any load greater
than a few k.
Opamp Circuit Diagram Symbol:
Basically an opamp is a differential voltage amplifier. It means it
amplifies the difference between input voltages V1 and V2. Here 3
situations are possible:
If V2 > V1 > Vo is positive
If V2 < V1 > Vo is negative
If
V2
Ideal Operational Amplifier:
V1,
Vo
The ideal opamp is a differential input, single ended output device. It has
the following characteristics
1. Infinite input resistance [Ri = Infinity]
2. Zero output resistance [RO = 0]
3. Infinite voltage gain [AV = Infinity]
4. Infinite bandwidth [BW = Infinity]
5. Infinite Common Mode Rejection Ratio
6. Infinite slew rate
7. Zero offset [ ie,V1 = V2 , VO =0]
8. Characteristics do not drift with temperature
Op amp Parameters:
Input offset voltage
Input offset current
Input BIAS current
Large signal voltage gain
Output voltage swing
Differential input resistance Ri
Input capacitance Ci
Common mode rejection ratio
Supply voltage rejection ratio
Slew rate
Gain bandwidth product
Input offset voltage
Input offset voltage is the voltage that must be applied between
the two input terminals of an op-amp to null the output.
Typical value of 741 IC is 6mV dc.
In the ideal op amp when both inputs are at zero volts the output
should be zero volts.
Due to imbalances within the device a small amount of voltage
will appear at the output.
This extra voltage can be eliminated by giving a small voltage
called Input offset voltage (VOS) to the amplifier.
Typically the input offset voltage for a 741 op-amp is around 1mV.
Input offset current
The algebraic difference between the current in the inverting
and non inverting terminal is known as the input offset
current Iio.
As the matching between two terminals
increases,the
difference between IB1 and IB2 become smaller.
Typical value for 741 IC is 200mA(max).
The input offset current is the difference between the two input
currents of the opamp with the output at zero volts.
Typically the input offset current for a 741 op-amp is 20 nA .
Input Bias Current:
IB is the average current flows in the Inverting and Non-Inverting
terminal of OP-AMP.
IB = (IB1 + IB2 )/2
Typical value for 741 is 500mA
The input bias current (IB) is the average of the currents enter into
the two input terminals with the output at zero volts.
Typically the input bias current is around 80nA.
This input bias current makes a voltage drop across the equivalent
source impedance seen from the input side of opamp.
Large Signal voltage Gain
It is the ratio of the output voltage and the differential input
voltage.
A= Output voltage/Differential input
= Vo/Vid
Typical value for 741 IC is 200,000.
Output Voltage Swing
This parameter indicates the values of positive and negative
saturation voltage of the op- amp.
For 741IC,it is +13 and -13V.
Differential Input Resistance Ri
Differential input resistance Ri is the equivalent resistance that
can be measured at either the inverting or non-inverting input
terminals with the other terminal connected to ground.
Typical value for 741 IC is 2 mega ohm.
Common-Mode Rejection Ratio:
When the same voltage is applied to both the input terminals
the voltage is called a common mode voltage Vcm and the opamp is said to be operating in the common mode configuration
CMRR is defined as the ratio of the differential voltage gain to
common mode gain.
CMRR = Ad/Acm
In OPAMP, the output voltage is proportional to the difference
between the voltages applied to its two input terminals.
When the two input voltages are equal ideally the output voltages
should be zero.
A signal applied to both input terminals of the opamp is called as
common-mode signal. Usually it is an unwanted noise voltage.
The ability of an op amp to suppress common-mode signals is
expressed in terms of its common-mode rejection ratio (CMRR).
Typically the CMRR for a 741 op-amp is around 90 dB.
CMRR=20 log10[Differential Voltage Gain/Common Mode Gain] dB
CMRR is defined as the ratio of the differential voltage gain Ad to the common
mode voltage gain ACM
CMRR = Ad / ACM.
For the 741C, CMRR is 90 dB typically. The higher the value of CMRR the better
is the matching between two input terminals and the smaller is the output
common mode voltage.
Supply voltage Rejection Ratio: (SVRR)
The change in an op-amps input offset voltage Vio caused by variations in
the supply voltage is called the SVRR.
It is expressed in microvolts per volt or in decibels.
SR= Vio/V
Typical value for a 741IC is 150V/V
SVRR is the ratio of the change in the input offset voltage to the corresponding
change in power supply voltages. This is expressed in V / V or in decibels,
SVRR can be defined as
SVRR = Vio / V
Where V is the change in the input supply voltage and Vio is the
corresponding change in the offset voltage.
For the 741C, SVRR = 150 V / V.
For 741C, SVRR is measured for both supply magnitudes increasing or
decreasing simultaneously, with R3 10K. For same OPAMPS, SVRR is
separately specified as positive SVRR and negative SVRR.
Large Signal Voltage Gain:
Since the OPAMP amplifies difference voltage between two input terminals, the
voltage gain of the amplifier is defined as
Because output signal amplitude is much large than the input signal the voltage
gain is commonly called large signal voltage gain. For 741C is voltage gain is
200,000 typically.
Slew Rate
The slew rate is the maximum rate of change of output voltage
for a step input voltage.
The slew rate makes the output voltage to change at a slower rate
than the applied input.
Eventually the output waveform is a distortion of the input
waveform.
The typical value for the slew rate is 0.5V/s.
Slew rate is defined as the maximum rate of change of output
voltage per unit of time and is expressed as volt per micro
second.
SR=(|dVo|/|ds|)max ie V\s
Gain Bandwidth Product
The gain bandwidth product(GB) is the bandwidth of the op-amp
when the voltage gain is 1.Typical value for 741 IC is 1MHz.
