Introduction / Context:
A Schmitt trigger inverter has two distinct thresholds and therefore naturally supports a relaxation oscillator when combined with a resistor and capacitor. This is a textbook way to produce a square wave with minimal parts for clocks, LED blinkers, and simple timing signals.
Given Data / Assumptions:
- Device is a Schmitt-trigger CMOS/TTL inverter.
- An R feeds the input node from the output; a C from input node to ground forms the timing network.
- Supply voltage is within the device’s operating range.
Concept / Approach:
The capacitor charges through R until the inverter input crosses V_T+; the output toggles LOW, causing the capacitor to discharge through R until the input crosses V_T−; the output toggles HIGH. The cycle repeats indefinitely, producing an oscillation whose period depends on R, C, and the hysteresis thresholds.
Step-by-Step Solution:
Wire inverter output to input through R; connect C from input to ground.On power-up, C charges toward the current output level through R.When V_in reaches V_T+, the inverter flips; C then discharges toward the opposite rail.Oscillation period approximates T ≈ k * R * C, where k depends on V_T+ and V_T−.
Verification / Alternative check:
Scope captures show a triangular waveform at the input node and a square wave at the output, confirming astable behavior.
Why Other Options Are Wrong:
Incorrect: Schmitt hysteresis specifically enables single-inverter oscillation.Only works with two cascaded inverters: A single Schmitt device suffices; two non-Schmitt inverters can also oscillate but need different biasing.True only at 12 V: Works across valid supply ranges; frequency varies with thresholds and supply.
Common Pitfalls:
Using a non-Schmitt inverter; without hysteresis the circuit may not start or may jitter.Choosing R or C values that violate input leakage or output drive limits.
Final Answer:
Correct
Discussion & Comments