Electronics Made Easy - a Complete Introduction to Electronics - Martin Denny [9]
Gain G = (Rf + Rin)/Rin, where Rin = Ri + Rs and R = Rf
In cases where a considerable variation in gain is required from a single stage amplifier problems can arise from the impedance imbalance caused by a potentiometer either in the feedback loop or at the input. This can result in excessive drift or instability of the output signal. These problems can be largely avoided if a proportional feedback circuit is used, see fig 11.
To calculate the maximum and minimum values of gain:
At Gain min: G = [(Rf + Rin)/Rin]*(R1 + Vr1 + R2)/(R2 + Vr1)
G = (Rf + Rin)(R1 + Vr1 + R2)/Rin(R2 + Vr1)
At Gain max: G = [(Rf + Rin)/Rin]*(R1 + Vr1 + R2)/R2
G = (Rf + Rin)(R1 + Vr1 + R2)/RinR2
Where R = Rf and Rin = Rs + Ri
The Parameters of Operational Amplifiers
Supply Voltage Range:
This is specified as ±V or +V1 to +V2 note there is always a minimum supply voltage.
Input Current:
This is the load current of the inverting and non-inverting inputs measured in nA (this is usually small and can be neglected in most designs).
Input Offset Voltage:
This is the standing voltage on the input which will be reflected as an offset voltage on the output, this is gain dependant.
Input Offset Current:
The input offset current will also be reflected on the voltage output it is input resistor and gain dependant.
Drift:
Input offset current and voltage dependant these figures are not always quoted for general purpose operational amplifiers.
Common Mode Rejection:
This is the noise rejection of the inputs where both inputs are subjected to the same noise. To minimise common mode noise rejection the input impedances of a differential amplifier should be balanced.
Series Mode Rejection:
This is the rejection of input noise in terms of current, to minimise the effects of series mode noise the input resistor impedances must be as small as possible.
Differential Input Voltage (max):
This is the maximum voltage swing the inputs can accept and is also limited by the supply voltage.
Power Dissipation (max):
The power dissipation of the operational amplifier is mainly affected by the output voltage swing and the output load (as the output impedance is small). The supply volts if significantly higher than the voltage swing will also be detrimental to the power dissipation.
Open Loop Voltage Gain:
This is the gain of the amplifier before external feedback components are added. It is usually quoted in db.
Note: Gain in db = 20logG* where Log10 = 1, Log100 = 2, Log1000 = 3,Log10000 = 4, etc where log = logarithm to base 10. Therefore a gain of 60db º 1000*.
Slew Rate:
This is the response of the output signal to a step change in the input signal measured in V/ms.
Output Voltage Swing (max):
This is quoted at a typical supply voltage for the device, and usually is approximately 2V below rail voltages.
Gain Bandwidth Product:
The frequency at which the output voltage is attenuated by 3db, with the input level constant.
The Operational Amplifier as a Summer:
If several signals are to be combined, ie summed these signals can be fed via their own input resistors to an operational amplifier, Fig 12 shows a non inverting amplifier with summing junction.
The gain is set by (Rf + R)/R, and 1/R = 1/(Rs1 + Ri1) + 1/(Rs2 +Ri2).to
+1/(Rsn + Rin) where n is the nth input.
The obvious disadvantage with this system is that to balance the input impedances increasingly high input resistors must be used limiting the available gain.
The Operational Amplifier as a Comparator
When the amplifier is used as a comparator the gain becomes the open loop gain of the device, as no feedback resistor is used. In this mode if the input voltage on the non-inverting + input is higher than the input on the inverting - input then the output voltage will rise to saturation, approximately 2 volts below the supply rail voltage. Conversely when the positive input is less than the negative input to amplifier out will drive negative.