Memo: Circuit 4

by: Laura Tilton & Sia Yiu

To: EE222 Students
From: Laura Tilton & Sia Yiu
Subject: Nifty Diode Circuit No. 4
Date: 10/23/95

The circuit we analyzed was nifty circuit No. 4 shown below as Diagram 1.

Diagram 1. Circuit diagram of nifty diode circuit no. 4

After we experimented with the circuit, we gave it a name, the dual clamp. This is because the circuit essentially "clamped" the input sinusoidal voltage at both a positive and negative voltage.

After building the circuit, we measured both Vin and Vout versus time on the oscilloscope. Graph 1 shows Vout with respect to time.

Graph 1. Vout vs. Time

We also plotted Vin and Vout with respect to time simutanously, making it easier to analyze this circuit. Graph 2 shows the two plots. The sin wave is Vin and the other one is Vout. We can see they overlap between the voltages 1.6 and -2.4 volts . Outside of this range, Vout is quite stable at 1.6 and 2.4 volts.

Graph 2. Vout vs. Time

Now we can visualize how this circuit works. Although Vout appears to trace Vin at certain values, it essentially plateus at a magnitudes much less than the magnitude of Vin. We will divide our analysis now into three sections in order to better explain w hat is physically happening in our circuit.

When Vin is greater than 1 volt, we can see that there will be a forward bias voltage applied to diode 1. Diode 2 (D2 in Diagram 1) on the other hand, basically acts as a open circuit since the diode is in reverse bias and no current will pass through it. Once Diode 1 has enough voltage to 'turn on', then the diode acts as a short. Vout, which measures the voltage drop across the branch, will be the 1 V DC source plus the voltage drop across the diode, which should be the 'turn on' voltage. This value for Vout should remain constant until Vin becomes small enough that Diode 1 is no longer in forward bias.

When Vin is less than -2 V, Diode 2 is in forward bias. Once there is enough voltage across the diode for it to conduct current (the turn on voltage of approximately .7 V) , the voltage across this branch will remain constant until this diode is no longer in forward bias. The current flowing is in the opposite direction as before, giving us a negative voltage drop.

Both of the magnitudes where Vout levels off can be broken down into two components, the DC source and the 'turn on voltage'. Simply, Vout for those two cases will always be the voltage drop for the respective branches. We see from graph 1 that when dio de one is in forward bias, the drop is approximately 1.65 V, while when diode 2 is in forward bias, the drop is -2.6 V. These figures confirmed what we predicted, that the magnitude of the clamping voltage is equal to the turn on voltage plus either 1 V o r 2V. We can attribute the slight bend in the 'clamped' region to the small slope of the I-V curve of the diode when in forward bias.

We have seen in this circuit yet another practical application of diodes. There are many uses for a "dual clamp" circuit like the one we have just assembled. For example if you had a system that was only allowed to have a input voltage with a limited ma gnitude. This circuit could act as a barrier of sorts, only allowing voltages less than a specific magnitude to ever be applied to the system. Another application could be for a system that needs an Input voltage that switches between two 'steady' volta ges over time. This may be of use to timing or switching systems. No matter what the application, this circuit has shown to be an interesting circuit in its own right, and lives up to the title "Nifty Diode Circuit".

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Last updated: Oct. 27, 1995
maintained by: Sia Yiu

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