# Simple version of an internal op-amp circuit

#### Kartschu Weger

Joined Jan 20, 2019
3
I'm currently learning how op-amps and transistor circuits work and want to understand the simple version of an internal op-amp circuit. However, there are still some open questions.

The first stage here is a differential amplifier with R3 as the constant current sink. There may also be a transistor current sink or current mirror when I'm not wrong. T3 is a level-shifter (common-emitter, voltage-amplification). It receives a small current from the collector of T1, which is proportional to the input difference (U_pos minus U_neg), right? That means T3 creates an inverted voltage on its collector (output), which is proportional to that current. (I'm not exactly sure as T3 may also use the voltage from T1 collector?). However, why not using an NPN transistor? T4 is a common collector circuit that provides a small output resistance and amplifies the current (power-amplification).
Is my understanding correct?

The resistors I have used to simulate are not proper as the output gets into the negative saturation when I insert a small AC signal on the pos. Input. How can I calculate the resistors so that the output has a good range between Ub+ and Ub-? How can this circuit be improved (I'm aware of adding a better constant current sink made of a transistor)? I also saw some capacitors sometimes, would that help? Maybe a capacitor in front of T3 to remove offset?

Thank you!

#### ericgibbs

Joined Jan 29, 2010
12,200
hi KW,
Have you considered adding some negative feedback from the output back to the input, in order to 'fix' the Gain of the amplifier.?
E

#### Kartschu Weger

Joined Jan 20, 2019
3
hi KW,
Have you considered adding some negative feedback from the output back to the input, in order to 'fix' the Gain of the amplifier.?
E
Hey,

yes I'm aware that a negative feedbacks from the output to the inverting input would make it better in terms of Gain and linearity. However, this is more for educational purpose and I just want to know how the circuit above exactly works (what T3 and T4 exactly do, if T3 is driven by the output current or output voltage from T1, how to calculate resistors here etc).

#### ericgibbs

Joined Jan 29, 2010
12,200
hi,
For reference only, this is your circuit in LTSpice, 40kHz sine at 10mV

E

Added a 2nd image, shows more detail.

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#### ericgibbs

Joined Jan 29, 2010
12,200
hi KW,
As you are investigating the design of an OPA circuit, consider what the steady state Vout should be for a zero volt Input.??
Normally it should be zero volts Output for zero volts Input signal.