Table of Contents
An Operational Amplifier Generally Consists of three stages
1. A Differential Amplifier
2. Additional Amplifier stages to provide the required voltage gain and dc level Shifting
3. An Emitter-follower or source Follower output stage to provide current gain and low output resistance.
In order to achieve the desired Low-frequency or dc gain of roughly 104 for a General-purpose op-amp, active load is preferred in the internal circuitry of the op-amp. Dual polarity supply voltage is required because the output voltage must be at ground when the differential input voltages are zero. A complementary push-pull emitter-follower or source follower output stage is used because an op-output amp’s resistance must be low. Additionally, a FET input stage is typically preferred because the input bias currents must be very small, on the order of picoamperes. A general op-amp circuit using JFET input devices is shown in the figure.
The M1 and M2 p-channel JFETs are used in the input differential amplifier stage. It uses a three-transistor active load made up of transistors Q3, Q4, and Q5. A two-transistor current source using PNP transistors Q6 and Q7 provides the stage’s bias current. R04’s indication of Resistor R1’s increased output resistance can be seen when looking into Q4’s collector. To provide bias current stability against transistor parameter variations, this is essential. Through Q5, the resistor R2 creates a clear bias current. The Q4 collector output is a single-ended output.
MOSFET‘s are used in place of JFETs with additional devices in the circuit to prevent any damage for the gate oxide due to electrostatic discharges.
The second stage, also known as the gain stage, employs the Q8 and Q9-formed Darlington transistor pair. With its emitter follower connection, the transistor Q8 offers high input resistance.
As a result, the loading effect on the input differential amplifier stage is reduced. For this stage, Q10 serves as an active load while transistor Q9 offers additional gain. The bias current for Q9 is established by the current mirror that Q7 and Q10 create. The voltage drop across R4 is created by the VBE drop across Q9 and the drop across R5, and this voltage determines the current flow through Q8. It can be set to a low value to make sure that Q8’s base current is also very low.
A class AB complementary push-pull output stage is the op-last amp’s stage. As an emitter follower, Q11 offers a high input resistance to lessen the effects of loading on the gain stage. The current mirror created by Q7 and Q12, through Q13 and Q14, provides bias current for Q11 in order to reduce cross over distortion. The two diodes can also be swapped out for transistors.
The overall voltage gain AV of the op-amp is the product of voltage gain of each stage as given by AV = |Ad | |A2 ||A3 |
Where Ad is the gain of the differential amplifier stage, A2 is the gain of the second gain stage and A3 is the gain of the output stage.