Operational Amplifier

Operational Amplifier (OA) is a high-integrated electronic component which is characterized by very high voltage amplification, high input resistance and low output resistance. OA is intended for work in frequency range from zero to its upper frequency limit, which is usually of the order of several MHz's. Because of these properties, the OA works with externally derived negative feedback and depending on the feedback character the OA can be used in configurations for modeling of different mathematical operations, in the devices for generating of signals with different waveform and time duration, or simply, for conditioning and shaping of the input signals.

In general, the operational amplifier structure consists of three basic stages:

I. Input stage - which is derived as differential amplifier which has a high input resistance;

II. Stage for voltage amplification - which can achieve extremely high amplification;

III. Output stage - which is realized as power amplifier providing low output resistance of the OA and output current from a few tenths of mA.


The operational amplifiers are usually produced in monolith bipolar or CMOS integrated technology, therefore their basic parameters and characteristics largely reflects the characteristics of the corresponding technology. The design of the devices with OA should consider the following characteristics:

1. The accuracy of the opened-branch amplification, the stability in terms of the temperature, the time and the power supply voltage;

2. The requirements about the power sources, the impedance of the signal source, the impedance of the load and the power dissipation;

3. The input voltage error and the polarization currents of the input stage, input and output impedance and their permanent changes as a result of aging process and temperature;

4. The frequency response, the transient response, the frequency stability, the ability for working with capacitive load and the ability for recovery after overloading;

5. The linear characteristic, distortions and the generated noise;

6. Factor of common signal rejection and power supply, the range of the input and output signals, the need of the input and output protection and error protection, the need of the external offset setting (at older components), etc.

Picture 1: Model of real Operational Amplifier


On the Picture 1 is shown a model of real operational amplifier for working mode with DC excitation signals. Here:

Avo - voltage amplification in open loop;
Ricm - input resistance for synphase (common) signal;
Rid - input resistance for differential signal;
Vos - not equality input voltage;
Ios - not equality input current;
C1 + C2 - input capacitance for differential signal;
C1C2/(C1 + C2) - input capacitance for synphase signal.


The basic parameter of the OA is the high coefficient of the voltage amplification in open loop, Avo. The voltage amplification is defined as ratio between the output and input voltage, Avo = Vo/Vi, under pre-defined values for temperature, power supply voltages and accurately determined load. The value of the amplification for the general-purpose operational amplifiers usually is specified at low frequencies and it moves in the range from 10^5 to 10^6. The tolerance of the voltage amplification value for these OAs is relatively high, so the minimal amplification can differ for few times the typical amplification. Also, the voltage amplification is frequency depended variable and in most of the cases it has at least one frequency dominant pole, which because of the high amplification is located in the low frequency area.