# Understanding the output waveform and using LTSpice

• Engineering

## Homework Statement:

Find the output waveform for the given circuit.

## Relevant Equations:

Not sure about which equations to use here (if any) For the following circuit I need to find the output waveform.

A. Conceptually I am at sea here but I will have a stab at it. My though process is as follows :
1. At a given instant, the AC voltage is at a fixed value and this value drops (##V_drop##) across the resistor ##R##.
2. The voltage (##V_drop##) across the diode and battery as well as the voltage across ##R_L## is constant for a given instant.
3. Current flow would be affected across the diode when the voltage across it is less than the cut-in voltage of the diode (0.7 V), however this information seems irrelevant in finding the output voltage.
4. At a given instant the AC source is the same as a DC source which is parallel to the voltage source ##V##. Two cases emerge here, one when the instantaneous voltage of the AC source is greater than ##V## and the other when the instantaneous voltage of the AC source is lesser than ##V##. In the former, V acts as a short (?) while in the latter the AC source can be neglected (?!).
5. Can't quite put a finger on how V affects the output waveform.
How do I arrive at the output waveform?!

B. I tried to use LTSpice to analyse the circuit, could you please tell me if I have replicated the circuit correctly? #### Attachments

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DaveE
Gold Member
Try to re-run your simulation with a larger magnitude sine wave input, let's say 50V. This will give you more insight. If Vout is less than about 10.7V then the diode will always be off.

You can assume that at any moment in time the diode will either be conducting, or not. So, you can start by analyzing the circuit in each of these two modes (i.e. one with a short circuit, one with an open circuit in place of the diode). Then think about when the circuit will switch between these two states.

• berkeman