Question Details

The steady state output (Vout), of the circuit shown below, will

Options

A

saturate to  -VEE

B

saturate to +VDD

C

become equal to -0.1 V

D

become equal to 0.1 V

Correct Answer :

saturate to  -VEE

Solution :

The correct answer is: saturate to -VEE

Step-by-Step Explanation:

1. Analyze the Circuit Diagram:
By inspecting the provided circuit diagram, we can identify the following components and connections:
- The circuit is configured as an inverting integrator using an operational amplifier (op-amp).
- A DC voltage source of value 0.1 V is connected to the inverting input terminal (-) via the resistor R1.
- A feedback capacitor C1 is connected between the inverting input terminal and the output node Vout.
- The non-inverting terminal (+) is connected to ground through a resistor R2.
- The op-amp is powered by positive and negative bias supply voltages, labeled as +VDD and -VEE respectively.

2. Apply Op-Amp Analysis Rules:
Assuming an ideal op-amp operating in its active/linear region initially:
- No current enters the input terminals of the ideal op-amp. Consequently, there is no voltage drop across resistor R2, which sets the voltage at the non-inverting terminal to:
V+=0 V
- By the virtual short concept, the voltage at the inverting terminal is equal to the non-inverting terminal:
V-=V+=0 V
Therefore, the inverting terminal acts as a virtual ground.

3. Calculate the Input Current:
The current Iin flowing from the 0.1 V DC source through the resistor R1 to the virtual ground is:
Iin=0.1-V-R1=0.1-0R1=0.1R1
Since no current enters the inverting terminal of the op-amp, all of this current Iin must flow through the feedback capacitor C1 towards the output:
IC=Iin=0.1R1

4. Determine the Output Voltage Behavior:
The current through the capacitor C1 is given by the relation:
IC=C1d(V--Vout)dt=-C1dVoutdt
Equating IC to the constant input current:
-C1dVoutdt=0.1R1dVoutdt=-0.1R1C1
Integrating with respect to time t gives:
Vout(t)=-0.1R1C1t+Vout(0)
This equation represents a continuously decreasing linear ramp voltage with a negative slope.

5. Steady-State Saturation:
As time t approaches infinity (t), the output voltage Vout(t) decreases without bound.
However, the output of any practical op-amp is limited by its power supply voltage rails. The negative limit of the output voltage range is defined by the lower supply voltage -VEE.
Thus, as the capacitor continues to charge up, the output voltage will eventually hit the negative limit and remain locked at that voltage.
Therefore, in the steady state, the output voltage will saturate to -VEE.

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