Diode: one-way traffic

A diode is an electrical component which  has zero or very low resistance in one direction and infinite or very high resistance in the other. This characteristic allows electric current to flow in one direction only, the forward bias direction, while blocking current in the opposite, the reverse bias, direction.



I tested this property of the diode experimentally by measuring the resistance of a 1N914 diode in either direction. In one direction the resistance was too high (> 10 MΩ) and the multi-meter was overload. In the other direction, the resistance was ~ 0.5 MΩ (it was varying a lot with touching, since the resistance of the diode is temperature dependent :).

However, the 1N914 diode is not ideal (i.e. it does not have 0 and infinite resistance in the two electrodes). The curve shows the I-V relationship of a typical silicon diode.
This shows that a minimum of ~ 0.6V has to be applied across a forward-biased diode to make current flow through it. It also shows that the voltage drop remains almost constant at ~ 0.6 - 1 V.
The I-V curve also shows that V/I or R, is close to 0 at V > 0.6V, and very high for negative voltage, which is characteristic of the diode.



Half Wave Rectifier 

A rectifier is an electronic device which converts alternating current (AC), which is periodically switching direction of flow, to direct current (DC), which flows in one direction only.

During each positive half-cycle  of the AC sine wave, the diode is forward biased and current flows through. During each negative half of cycle, the diode is reverse biased and no current flows through it. This results in a Vout across the load R equal to Vs - VDiode for the positive half of the cycle, and 0 on the negative half.

HH Lab 3-2

I constructed the above half-wave rectifier circuit with a 2.2 kΩ resistor. The output was as expected, a half-wave rectified 60 Hz sine-wave. When the diode was connected in the forward-biased direction, the negative part of the 60 Hz AC could not get through, while when it was connected in the reverse-biase direction, the positive part was omitted as shown.

I wasn't being clever at all when taking these pictures, so I'll mention a few subtle details which are not obvious from these snapshots.

A smarter photographing would have shown this:








Vout = Vs - VDiode = 1.3 V - 0.6 V = 0.7 V
Period of 60 Hz AC ~ 16.7 msec
So, the period of half-wave is ~ 8.3 ms. However, the width of the rectified positive part of wave is shorter because no current flows when Vs < 0.6 V.

The second step towards AC-DC conversion is to smooth out the ripples. This can be done by adding a capacitor parallel to the resistor, which increases the "RC" time lag. If R is large enough so that RC >> 1/120 sec, then the capacitor will not discharge very much before it starts charging again, and the ripple will be small.

Here, I connected a 47 μF capacitor parallel to the 2.2 kΩ resistor, with Vs of 1.3 V.
Ripple amplitude is given by:

ΔV = 0.7V (1/60 x 2.2kΩ x 47μF) ~ 0.1 V.
This is what is seen in the scope :)

Signal Diodes HH Lab 3-5

I built the following circuit, which is a rectified differentiator, and drove it with a 10 kHz square wave.
The results obtained made sense, since it was a reminder of the square wave differentiated in last lab. However, this time the spikes did not appear in both direction, but only in the positive direction. This is expected since this is a rectified square wave!

Now, on zooming in closer, it can be seen that these "spikes" are RC discharge curves.

The signal on the scope is decaying with time constant, ~350ns.
R = t/C = 350ns/560pF ~ 600 Ω

If 1k and 2.2k are in parallel, effective R

R = 2.2/3.2 ~690 Ω

So I guess the two resistors are in parallel.
When I take the 2.2k out, the new time constant (I'm not too confident on this since I ddn't take a proper measurement) ~ 500ns
This corresponds to an R of ~ 0.9 k
On taking the 2.2k out, the 1 Meg of the scope kicks in. 
R = 1000/1001~ 1k

Makes somewhat sense, though I'm not entirely confident on the calculation.

Diode Limiter HH Lab 3-7

The diode can also be used to limit input voltages to < 0.6 V, which is useful for certain instruments which are sensitive to high voltages. The following circuit is called a diode limiter. If we remember the I-V curve at the top of the page, after crossing 0.6V barrier, the voltage drop across the diode becomes relatively constant - diode limiter takes advantage of this property. On driving it with sine, triangle and square wave, the following results were obtained:

As we can see, in all of the above cases, the Vout stays put at 0.6 V after we have crossed the point. It is also interesting to note how the triangle and sine wave changes their shape as Vin is increased. The edges of the wave rounds off, and they move towards becoming square waves. This is because signal over 0.6V is getting cut-off.

Impedances of Test Instruments

Ideal voltage measuring devices should have very high input impedance so as not to perturb the circuit. The input impedance of a scope comes from a 1MΩ  resistor in parallel with a 30 pF capacitor. The 30 pF is called stray capacitance since it is not put there intentionally. For a DC signal, this arrangement is fine since the capacitor provides infinite impedance. However, for high frequency signals the situation gets hairy as ZC gets smaller. We want ZC >> ZSignal under all circumstances, and hence special care has to be taken when dealing with high frequency sources.

To test the internal resistance of the scope, I connected a 1MΩ in series with the scope, driving it with a 10Vpp signal at 100 Hz. The output became halved as expected, showing that the internal resistance is also 1 MΩ.

On raising the frequency to a few kHz, the voltage drop increases further, and phase shift can be seen, show the effects of the stray capacitance. At 5 kHz, the Vpp is 2.8 V. Using this information, the capacitance of the circuit can be measured.

(Vin/Vout)^2 = 1+(ωRC)^2

(10.2/2.8)^2 - 1 = (5kHz x 1k x C)^-2

C ~ 10^-10 F ~ 100 pF

This is a bit bigger than I anticipated, maybe the capacitance from the cable adds up.
Also, I think I need to put in a factor of 2pi with the frequency, which will make it even bigger, almost a nF :(

A capacitor needs to be added in parallel to make this circuit work as a divide-by-two signal attenuator at all frequencies???