Ideal Operational Amplifiers
As well as resistors and capacitors, Operational Amplifiers, or Op-amps
as they are more commonly called, are one of the basic building blocks of Analogue Electronic Circuits.
Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification
and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add,
subtract, integration and differentiation.
An ideal Operational Amplifier is basically a three-terminal device which consists of two high
impedance inputs, one called the Inverting Input, marked with a negative or "minus" sign,
( - ) and the other one called the Non-inverting Input, marked
with a positive or "plus" sign ( + ).
The third terminal represents the op-amps output port which can both sink and source either a voltage or
a current. In a linear operational amplifier, the output signal is the amplification factor, known as the amplifiers gain
( A ) multiplied by the value of the input signal and depending on the nature of these
input and output signals, there can be four different classifications of operational amplifier gain.
- Voltage Voltage "in" and Voltage "out"
- Current Current "in" and Current "out"
- Transconductance Voltage "in" and Current "out"
- Transresistance Current "in" and Voltage "out"
Since most of the circuits dealing with operational amplifiers are voltage amplifiers, we will limit the tutorials
in this section to voltage amplifiers only, (Vin and Vout).
The amplified output signal of an Operational Amplifier is the difference between the two signals being applied
to the two inputs. In other words the output signal is a differential signal between the two inputs and the input stage of
an Operational Amplifier is in fact a differential amplifier as shown below.
The circuit below shows a generalized form of a differential amplifier with two inputs marked V1
and V2. The two identical transistors TR1 and TR2 are
both biased at the same operating point with their emitters connected together and returned to the common rail,
-Vee by way of resistor Re.
The circuit operates from a dual supply +Vcc and -Vee which
ensures a constant supply. The voltage that appears at the output, Vout of the amplifier is the difference
between the two input signals as the two base inputs are in anti-phase with each other. So as the forward bias of transistor,
TR1 is increased, the forward bias of transistor TR2 is reduced and vice versa.
Then if the two transistors are perfectly matched, the current flowing through the common emitter resistor, Re
will remain constant.
Like the input signal, the output signal is also balanced and since the collector voltages either swing in opposite
directions (anti-phase) or in the same direction (in-phase) the output voltage signal, taken from between the two collectors is, assuming
a perfectly balanced circuit the zero difference between the two collector voltages. This is known as the Common Mode of Operation
with the common mode gain of the amplifier being the output gain when the input is zero.
Ideal Operational Amplifiers also have one output (although there are ones with an additional differential output)
of low impedance that is referenced to a common ground terminal and it should ignore any common mode signals that is, if an identical signal
is applied to both the inverting and non-inverting inputs there should no change to the output. However, in real amplifiers there is
always some variation and the ratio of the change to the output voltage with regards to the change in the common mode input voltage is
called the Common Mode Rejection Ratio or CMRR.
Operational Amplifiers on their own have a very high open loop DC gain and by applying some form of
Negative Feedback we can produce an operational amplifier circuit that has a very precise gain characteristic
that is dependant only on the feedback used. An operational amplifier only responds to the difference between the voltages on
its two input terminals, known commonly as the "Differential Input Voltage" and not to their common potential. Then if
the same voltage potential is applied to both terminals the resultant output will be zero. An Operational Amplifiers gain is
commonly known as the Open Loop Differential Gain, and is given the symbol (Ao).
Equivalent Circuit for Ideal Operational Amplifiers
Op-amp Idealized Characteristics
|Open Loop Gain, (Avo)
||Infinite - The main function of an operational amplifier is to amplify the
input signal and the more open loop gain it has the better. Open-loop gain is the gain of the op-amp without positive
or negative feedback and for an ideal amplifier the gain will be infinite but typical real values range from about
20,000 to 200,000.|
|Input impedance, (Zin)
||Infinite - 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). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps.|
|Output impedance, (Zout)
||Zero - 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. Real op-amps have output-impedance in the 100-20Ω range.|
||Infinite - An ideal operational amplifier has an infinite frequency response
and can amplify any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have an infinite
bandwidth. With real op-amps, the bandwidth is limited by the Gain-Bandwidth product (GB), which is equal to the
frequency where the amplifiers gain becomes unity.|
|Offset Voltage, (Vio)
||Zero - 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.|
From these "idealized" characteristics above, we can see that the input resistance is infinite, so
no current flows into either input terminal (the "current rule") and that the differential input offset
voltage is zero (the "voltage rule"). It is important to remember these two properties as they will help us understand
the workings of the Operational Amplifier with regards to the analysis and design of op-amp circuits.
However, real Operational Amplifiers such as the commonly available uA741,
for example do not have infinite gain or bandwidth but have a typical "Open Loop Gain" which is defined as the amplifiers
output amplification without any external feedback signals connected to it and for a typical operational amplifier is about
100dB at DC (zero Hz). This output gain decreases linearly with frequency down to "Unity Gain" or 1, at about 1MHz and this
is shown in the following open loop gain response curve.
Open-loop Frequency Response Curve
From this frequency response curve we can see that the product of the gain against frequency is
constant at any point along the curve. Also that the unity gain (0dB) frequency also determines the gain of the
amplifier at any point along the curve. This constant is generally known as the Gain Bandwidth Product or
Therefore, GBP = Gain x Bandwidth or A x BW.
For example, from the graph above the gain of the amplifier at 100kHz = 20dB or 10, then the
GBP = 100,000Hz x 10 = 1,000,000.
Similarly, a gain at 1kHz = 60dB or 1000, therefore the
GBP = 1,000 x 1,000 = 1,000,000. The same!.
The Voltage Gain (A) of the amplifier can be found using the following formula:
and in Decibels or (dB) is given as:
An Operational Amplifiers Bandwidth
The operational amplifiers bandwidth is the frequency range over which the voltage gain
of the amplifier is above 70.7% or -3dB (where 0dB is the maximum) of its maximum output value as shown below.
Here we have used the 40dB line as an example. The -3dB or 70.7% of Vmax down point from the
frequency response curve is given as 37dB. Taking a line across until it intersects with the main GBP curve
gives us a frequency point just above the 10kHz line at about 12 to 15kHz. We can now calculate this more
accurately as we already know the GBP of the amplifier, in this particular case 1MHz.
Using the formula 20 log (A), we can calculate the bandwidth
of the amplifier as:
37 = 20 log A therefore, A = anti-log (37 ÷ 20) = 70.8
GBP ÷ A = Bandwidth, therefore, 1,000,000 ÷ 70.8 = 14,124Hz,
Then the bandwidth of the amplifier at a gain of 40dB is given as 14kHz as previously predicted
from the graph.
If the gain of the operational amplifier was reduced by half to say 20dB in the above frequency
response curve, the -3dB point would now be at 17dB. This would then give the operational amplifier an overall gain of
7.08, therefore A = 7.08.
If we use the same formula as above, this new gain would give us a bandwidth of approximately
141.2kHz, ten times more than at the 40dB point. It can therefore be seen that by reducing the overall "open loop gain"
of an operational amplifier its bandwidth is increased and visa versa. In other words, an operational amplifiers bandwidth
is proportional to its gain. Also, this -3dB point is generally known as the "half power point", as the output power of the
amplifier is at half its maximum value at this value.
Operational Amplifiers Summary
We know now that an Operational amplifiers is a very high gain DC differential amplifier
that uses one or more external feedback networks to control its response and characteristics. We can connect external resistors
or capacitors to the op-amp in a number of different ways to form basic "building Block" circuits such as, Inverting, Non-Inverting,
Voltage Follower, Summing, Differential, Integrator and Differentiator type amplifiers.
An "ideal" or perfect Operational Amplifier is a device with certain special characteristics such as infinite
open-loop gain Ao, infinite input resistance Rin, zero output resistance
Rout, infinite bandwidth 0 to ∞
and zero offset (the output is exactly zero when the input is zero).
There are a very large number of operational
amplifier IC's available to suit every possible application from standard bipolar, precision, high-speed, low-noise,
high-voltage, etc in either standard configuration or with internal JFET transistors. Operational amplifiers are
available in IC packages of either single, dual or quad op-amps within one single device. The most commonly available
and used of all operational amplifiers in basic electronic kits and projects is the industry standard μA-741.
In the next tutorial about Operational Amplifiers, we will use
negative feedback connected around the op-amp to produce a standard closed-loop amplifier circuit called an
Inverting Amplifier circuit that
produces an output signal which is 180o "out-of-phase" with the input.