On most circuit designs, the op-amp is one of the common components used. Op-amps or operational amplifiers are linear devices that have properties vital for DC amplification. Op-amps are commonly used extensively in signal conditioning, filtering and to perform mathematical operations such as addition, subtraction, integration and differentiation. Common circuits where operational amplifiers are used are buffers, amplifiers, preamplifiers, and equalization circuits.
When designing a circuit around an op-amp, it is good to be familiar with its characteristics. By knowing the op-amps characteristics, it is easier to understand and add the correct components around it. To help circuit designers and enthusiasts, rules have been developed in designing circuits using an op-amp based on their characteristics. Such rules are commonly known as the “Golden Rules”.
Here are the golden rules of operational amplifiers:
1) Infinite Open Loop Gain
Open loop gain is the gain of the op-amp without positive or negative feedback. Ideally, the open loop gain of an op-amp will be infinite but typical real values range from about 20,000 to 200,000.
In most cases, the open loop gain characteristic of an op-amp is not taken into consideration when designing circuits. But when dealing with high precision circuits, this must be given more attention. Open loop gain impacts DC accuracy and the gain error of your circuit. Open loop gain also impacts gain bandwidth product.
A typical rule of thumb is the higher the open loop gain the better the performance for your circuit.
2) No current flowing through both of the Inputs
The input impedance of an op-amp, is the ratio of the input voltage to the input current and is assumed to be infinite. With this very high input impedance, any current flowing from the source supply is prevented from entering into the amplifier's input circuitry. Although ideally it is assumed that the input impedance of an op-amp is infinite and has zero current flow into the inside, real op-amps have input leakage currents from a few pico-amps to a few milli-amps.
As you may have observed in op-amp lessons, this characteristic is used in deriving gain formulas of different op-amp configuration.
3) Potential Difference between input pins is ZERO
Negative feedback is the process of coupling the output back, in such a way to cancel some portion of the input signal. In return, our amplifier improves in characteristics such as linearity, flatness of response, and predictability.
When negative feedback is added to an op-amp, the input pins become identical. Meaning, whatever is the voltage present in the non-inverting input is also present in the inverting input.
In the example inverting op-amp configuration below, we can see that the non-inverting input is connected to ground. The non-inverting input is now set to 0V, meaning that the inverting input is also at 0V.
Another example is a non-inverting op-amp configuration with a bias voltage applied to its non-inverting input. The non-inverting is biased by a voltage divider network, biasing the non-inverting input at half of VCC. This means that the voltage at the inverting input is also equal to half of VCC.
This op-amp characteristic can be used practically when checking an op-amp whether it is still good or has gone bad. You can construct a single supply buffer circuit using the op-amp to be tested. Since it’s a single supply configuration, a virtual ground must be established. This is simply done by having a voltage divider at the non-inverting input of the op-amp, see circuit below. After constructing the circuit, measure the voltage levels at both input terminals of the op-amp, they should read the same or close. In this case the voltage levels at the input should be close to 4.5V. If the voltages at the input are not close or equal to each other, you may have a bad or damaged op-amp or maybe you just constructed the circuit wrong so you may want to double check on that first.
As a summary, here are the “golden rules” of op-amps:
- The op-amp has an infinite open loop gain. Ideally, this means that any voltage differential on the two input terminals will result in an infinite voltage on the output. But in real op amps, the output voltage is limited by the power supply voltage. Because the output voltage can’t be infinite, the gain can’t be infinite either.
- There is no current flowing through either of the inputs of the op-amp. This also means that it doesn’t load the driving source and does not affect the input voltage.
- In a circuit with negative feedback, the potential difference between the inverting and non-inverting inputs is zero. Adding a negative feedback to an op-amp circuit stabilizes the op-amp’s characteristics. Also, with a negative feedback, the gain of an op-amp is controlled and calculated.
Now that you have learned these, you can be a better circuit designer using op-amps and also be better in troubleshooting circuits involving op-amps.
