Topic: Analog Circuits
The Lower Threshold Point (LTP) voltage of the Schmidt trigger with an ideal op-amp shown in Figure is
(Figure shows an inverting Schmitt trigger with $V_{in}$ at the inverting terminal, feedback from $V_{out}$ to the non-inverting terminal via a 10 k$\Omega$ resistor, and a 5 k$\Omega$ resistor from the non-inverting terminal to ground. Power supply is $\pm 5V$.)
- A. 5 V
- B. -5 V
- C. 1.667 V
- D. -1.667 V
Answer: D. -1.667 V
Explanation: This is an inverting Schmitt trigger. The reference voltage at the non-inverting terminal $V^+$ is determined by the voltage divider formed by the 10 k$\Omega$ and 5 k$\Omega$ resistors:
$V^+ = V_{out} \times \frac{5k\Omega}{10k\Omega + 5k\Omega} = \frac{1}{3} V_{out}$.
The output $V_{out}$ switches between saturation voltages $+5V$ and $-5V$.
The Lower Threshold Point (LTP) occurs when $V_{out}$ is at its low state ($-5V$).
$V_{LTP} = \frac{1}{3} \times (-5V) = -1.667 V$.