Real op amps are not ideal. They have limitations. To understand and discuss the origins of these limitations, see the simplified op amp circuit diagram shown in Figure
Although simplified, this circuit contains the three basic elements normally found in op amps:
- Input stage
- Second stage
- Output stage
The function of the input stage is to amplify the input difference, Vp – Vn, and convert it to a single-ended signal. The second stage further amplifies the signal and provides frequency compensation.
The output stage provides output drive capability.
Input Stage of Simplified Op Amp Circuit Diagram
Symmetry of the input stage is key to its operation. Each transistor pair, Q1-Q2 and Q3-Q4, is matched as closely as possible.
Q3 is diode connected. This forces the collector current in Q3 to equal IC1. The base-emitter junctions of Q3 and Q4 are in parallel so they both see the same VBE. Because Q4 is matched to Q3, its collector current is also equal to IC1. This circuit is called a current mirror.
Current source 2IE is divided between Q1 and Q2. This division depends on the input voltages, Vp and Vn. When Vp is more positive than Vn, Q1 carries more current than Q2, and IC1 is larger than IC2. The current mirror action of Q3-Q4 causes IOUT1 to flow into the collector-collector junction of Q2-Q4.
When Vn is more positive than VP, Q2 carries more current than Q1 and IC2 is larger than IC1. The current mirror action of Q3-Q4 causes IOUT1 to flow out of the collector-collector junction of Q2-Q4.
IOUT1 is the single-ended signal out of the first stage and isproportional to the differential input, Vp – Vn. IOUT1 = gm1(Vp – Vn). The term gm1 is called the transconductance of the input stage. The input stage is a transconductance amplifier.
Second Stage of Simplified Op Amp Circuit Diagram
The second stage converts IOUT1 into a voltage and provides frequency compensation. If IOUT1 flows into the collectorcollector junction of Q2-Q4, the second stage output voltage is driven positive. If IOUT1 flows out of the collector-collector junction of Q2-Q4, the second stage output voltage is driven negative. The second stage is a transresistance amplifier.
The capacitor, CC, in the second stage provides internal frequency compensation. It causes the gain to role off as the frequency increases. Without CC, external compensation is required to prevent the op amp from oscillating in most applications.
Output Stage of Simplified Op Amp Circuit Diagram
The output stage is a typical class AB, push-pull amplifier. The emitter follower configuration of Q6 and Q7 provides current drive for the output load, with unity voltage gain. The output stage is a current amplifier.