by Harry Lythall - SM0VPO


Over the years there has been the occasional posts to my messageboard, and quite lot of questions have been received by e-mail regarding the use of diodes for switching, as opposed to using relays. This document is a theoretical treatment of diode switching.

In this article I will cover basic uses, device selection, AF switching, and RF "steering". I will also present a couple of novel uses for diodes in their use as a switch. I will, of course, include the PIN diode, towards the end.

The document is NOT a practical project, although many of the circuits can be constructed and used as presented. There are no PCB foils. The intercom, for example, was constructed in the 80s for a friend when he was newly married. He was delighted with the performance of it.

Introducing the Diode

Almost ALL diodes will pass current in one direction, but when the voltage polarity is reverse the diode does not conduct. You can measure this with an Ohm-meter (resistance meter). The Cathode (K) usually has a band around the body of the diode.

at (a) no current flow, but at (b) current does flow

The primary use is a rectifier, or detector, for Alternating Current. Early diodes consisted of a heated Cathode inside a tubular Anode. Typical devices were EB91, and EZ81, each of which contained two diodes and had a 6.3v heater, isolated from the cathode. Then came the Germanium Point Contact diode, which replaced the vacuum tube because it was really effecient, especially for small-signal applications. A typical Germanium diode is the OA91, which is still used in manny applications and older designs today. For higher power levels the Silicon diode cannot be beat for low frequency applications. A typical smal-signal Silicon diode is the 1N914 and a typical power rectifier diode is the 1N4001.

A typical point-contact germanium diode

Typical silicon switching diodes

A typical silicon rectifier diode

All these diodes have different properties. I will forget about the vacuum valve (tube) diode since we will not be using them in modern circuits. In principle, the silicon and germanium diode have slightly different properties:

Point Contact - Germanium
These have very low capacitance between the two electrodes. This makes them ideal for detecting radio frequencies. Early diodes manufactured in the 60s are still used today.
Forward resistance is typically 1 Ohm.
Reverse resistance is typically 1MOhm.
Forward voltage drop typically 0.15v.

Junction Diode - Silicon
These have capacitance between the two electrodes either side of the junction, and they are not so goodt as a signal detector.
Forward resistance is typically 0.5 Ohm.
Reverse resistance is typically >100KOhm.
Forward voltage drop typically 0.75v.

Zener Diode - Silicon
These have HUGE capacitance between the two electrodes. They are heavily "doped" so that when non-conducting they break-down when a particular voltage is reached. When the voltage is reduced below this "avalanch point" the diode recovers again. Zener diodes make perfect Variable-Capacitance diodes ;-)
Forward resistance is typically 0.5 Ohm.
Reverse resistance is typically >100KOhm.
Forward voltage drop typically 0.75v.

By the way, did you know that all diodes and bipolar transistors are sensitive to light? The power diodes are encapsulated in plastic, so they are shielded from light, but those in glass envelopes are not. If you get terrific hains "hun" in your DC receiver, using a diode bridge product detector, then try again in darkness - switch out the lights. Neon tubes kick out a terific light "buzz".

A simple Switch

Given that the diode can be made to conduct, or not to conduct, simply by the choice of applied voltage polarity, then it must be possible to use any diode as a switch. So if you want to follow me, then just wire together the following circuit on a bit of breadboard, or something. You could draw out the circuit and tape the components to the paper, then connect them as shown:

Experiment #1

Set the signal generator to 1v RMS (3v peak-to-peak) at 1kHz. Set the oscilloscope to 1v/cm and set the timebase to 200uS/cm (0.2uS/cm). You will see an undistorted sinewave on the scope, and having an amplitude of 3 divisions.

Now disconnect the 9v battery, and reconnect it but with the polarity reversed. The signal will dissapear from the oscilloscope. You have built a diode switch.

Experiment #2

Replace the 9v battery with a 1.5v AA cell. With the battery polarity as shown in the diagram, the waveform should still be an undistorted sinewave. But when the battery is reversed you will see just small pulses - nothing like a sinewave.

This demonstrates that the signal waveform must be lower than the switching voltage. For the switch to work perfectly the voltage must be a minimum of 2.2v - 1/2 of the 3v pk-to-pk plus the 0.7v needed to make the diode conduct.

Experiment #3

Repeat experiment 1, but connect a DC voltmeter across the diode terminals. The meter should have a sensitivity of 20kOhms/volt and be set to the 10v range. Ideally you should use a 10v-0v-10v center-reading meter, but you can use an ordinary meter and swap the leads to get a positive reading.

When the circuit was connected as per the diagram, the voltage reading was 0.7vDC. This is because the diode was conducting and the 0.7v is the diode forward bias voltage-drop. When the battery was reversed, the voltage reading was 10v. The diode is open-circuit and the full battery voltage is across the diode.

A Practical Application - Intercom

The simple switch shown so far can be used in simple audio applications. Here is a simple "all-master" intercom circuit that steers an audio amplifier towards one of several distant receivers. All units shown are identical and are connected to the same multi-pair cable. One battery powers all units.

5-station "all master" intercom using diode switching.

You press the button corresponding to the station to whom you want to talk. When any "TALK" switch is pressed, the horisontal diodes feed 12v DC power from the cable to the MIC amplifier 741. Here you can see how diodes are used to combine the DC to a common source. This prevents power being fed back to the other channels.

The 1K0 resistors feed the DC power to one of the four selected diodes. The DC operating point of the 741 OpAmp is about 5.5v, which is applied to the cathode of all four vertical diodes. The TALK switch and associated resistor causes one of these four diodes to be positively biased. The diode will clamp the selected station's line to 6.2v, with 500mV RMS aoudio superimposed upon this. In this circuit you can see that station 2 button has been pressed.

At station 2, the DC and audio signal is passed through a chocke to remove unwanted RF. The 2K2 resistor clamps the input to ground in the absence of any input signal. When called, the 2K2 is too high a value to affect this. The receiver circuit is a DC-coupled audio amplifier. When not called the DC conditions are upset and it draws no current. When called, the correct DC bias is received from the calling station and the amplifier drives the speaker.

In operation you just press the button of the station you wish to speak to. That person presses your station button to speak back. This method means that the other stations are also free and can also have a private conversation: for example stations 1 to 2, and another conversation between stations 3 and 4.

The OpAmp MIC amplifier has a pair of back-to-back diodes to limit the audio signal to 1.5volts peak-to-peak, which prevents the diode switching being overloaded. The 220R resistor in the feedback circuit is selected for correct gain. I have assumed that an ELECTRET mic is used and has a 10mV to 20mV output. If you use a dynamic mic then you need to remove the marked 10K resistor, and perhaps select the 220 Ohm for more gain (lower the value). The speaker and MIC should ideally be placed as far apart as possible, and pointing away from each other. Avoid mechanical conduction between these two. In normal operation you will need to speak close to the MIC, and the 220 resistor should be set so that feedback does not occur.

The speaker impedance should be 36 Ohms.

A Practical Application - Frequency Modulation

Here is a rather novel application. Most of you will probably recognise the oscillator circuit as a colpits oscillator. The "interesting" bit is the diode and potentiometer. Here is the circuit for your perusal:

In this case the frequency of the oscillator is determined by the values of the capacitor and inductor: C and L. Notice that I have now added a diode and another capacitor 10C (10 times bigger than C). The oscillations across the ends of L are a sinewave, varying above and below the Gnd (battery -ve) terminal.

I have connected a diode to the end of L, and coupled this to a potentiometer. The pot provides a variable voltage, from 0 to +3v in my example. As soon as the oscillations exceed the potetiometer voltage, then capacitor 10C is switched in circuit, extending the period time of the oscillations. I chose 3v because in the last oscillator I measured the level was +/-3v from peak-to-peak.

The overall effect is to lower the oscillator frequency by changing VR1. Maximum voltage should be equal to no frequency shift. This technique can give a VERY wide frequency shift, although distortion is present due to an asymetric waveform. The circuit does therefore need some form of post filtering, but since there should also be a buffer then the addition of a low-pass filter should be easily implemented.

Shunt Diodes

So far I have covered series-pass diodes, and how they can be turned on and off by the passage of current. If you go back to experiment #1, but this time turn the generator up to 1MHz, you will see that the circuit does not completely switch OFF the signal when the battery is reversed. This is because there is some signal "hop-over" through the diode. This is due to magnetic and capacitive coupling. Consider this experiment.:

Modified experiment, with shunt diode D2.

Here we see the same circuit as before, but another resistor and diode has been added in the signal circuit. Again, the capacitors are "sprinkled in" when needed to block the DC switching voltages.

When the battery is connected as shown, the battery positive is fed through a resistor to the Anode of D1, making it conduct. Another resistor feed the positive to D2 Cathode, preventing D2 from conducting. But when the battery is reversed, D1 will stop conducting, breaking the signal path. D2 will also conduct, acting to short any "hop-over" signal to ground. In this way, the combination of series and shunt diodes can improve the isolation. This sort of isolation is necessary when dealing with VHF transmitters and sensitive receivers.

RF Switching

When switching an antenna between a transmitter and receiver, the 1N914 or 1N4148 can still be used, but with decreasing success as the frequency rises. It is common practice in VHF transcievers to use a PN junction diode, but with a lightly doped "Intrinsic" region between the P and N layers. The diode is called a PIN diode.

When the PIN diode is made to pass a current it has a low resistance, just like a normal PN diode. But when the curren is removed, there is a delay when charge carriers do not immediately recombine, resulting in a delay. This delay gives causes a phase shift and can give the PIN diode many other uses, such as police RADAR speed detector jammers. But the "more perfect diode" properties and OFF-delay are the ones relied upon to protect your nice 0.1uV sensitivity receiver from getting a 100-volt jolt.

Here is a basic diode T/R switching circuit:

When switch S1 is open, both D1 and D2 are not biased at all, so the antenna signal is passed through the 1/4-wave line to the receiver. When S1 is closed, current passes through RFC1, D1, and RFC2, so D1 effectively passes RF from the transmitter to the antenna.

Current will also pass through RFC3 and diode D2, so D2 will shunt any RF at the input of the receiver to ground. The dead short across the receiver input is reflected to the transmitter as a high impedance by the 1/4-wave line. In this way the diode D2 will not shunt the transmitter/antenna circuit.

This technique can be built up to quite a complex arrangement requiring many diodes, and capable of handling 100s if not 1000s of watts, and at frequencies in the GHz region.

I hope that you have learned something useful with this information. Very best regards from Harry - SM0VPO

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