BASIC 
ELECTRONICS COURSE 
Page 33 INDEX

CONNECTING AN "INPUT DEVICE" TO A CIRCUIT
One of the most important things that any circuit designer will have to do is connect a "device" to the input of a circuit. 
A "device" is any item that picks up information from the outside world and produces a signal or a voltage. A "device" can also be called a "pick-up" or "input device" and it may be a coil, photo transistor, reed switch, ordinary switch, touch switch, Hall effect device, set of water probes, microphone, piezo diaphragm, timing circuit, solar cell, light-dependent resistor (LDR), thermal sensor, or one of a dozen other items. 
Most of the time you cannot connect a device to a circuit and expect it to work. Sometimes you may be lucky but to get it to work properly it must be matched to the circuit. By this we mean the output impedance of the device must match the input impedance of the circuit. 
The output IMPEDANCE of a device can also be called its RESISTANCE.  But the term impedance takes into account the fact that the resistance changes according to the frequency at which the device works. This generally means the value of impedance will be higher than the DC resistance. The DC resistance is the value obtained from a multimeter.  

But this is getting too technical. 
Let's keep it simple. 
There are basically two types of devices:
1. Devices that require a voltage to be applied to them and they produce a waveform when they are operating.
2. Devices that generate a waveform when they are moved (hit or vibrated) or when a magnet is placed nearby.
The end result is the same. A VOLTAGE (in the form of a WAVEFORM) is DELIVERED (outputted) by the device. 
To keep the discussion simple, we will consider this waveform to have very little current.  In other words, if the waveform is 2v, you cannot connect a globe and expect it to illuminate. The voltage can only be detected (read) by very sensitive equipment (such as a multimeter).
If you connect a globe and try read the voltage, it will be almost zero because the globe puts a heavy load on the circuit. If the circuit can only deliver 1mA, the voltage rises until 1mA flows through the globe and does not rise further. The result may be a voltage of 3mV!

This is exactly what happens if you connect a "pick-up" to a circuit without knowing a few technical facts. If the matching is far from perfect, a "device" may have an output of 100mV when measured with a multimeter, but the output may be reduced to 10mV or less if connected to the wrong type of circuit.

So, what's the solution?
The solution is to use an amplifier circuit that will not load the "device."  A simple common-emitter configuration is ideal. To match the output impedance of the device to the input impedance of the amplifier, a very simple component is fitted - a capacitor! 
The capacitor performs three functions:
1. It separates the DC voltage on the "device" from the DC voltage on the input of the amplifier. 
2. It allows only the AC waveform to pass from the "device" to the amplifier.
3. It "matches" the impedance (resistance) of the "device" to the input impedance of the amplifier.

 
In simple terms: "FIT A CAPACITOR"  and all your problems are solved.
The amazing thing is the capacitor does not have to be an exact value. In most cases a 10n will work as well as 22n or 100n for audio. If the frequency is say ten times higher than audio, the capacitor will have to be reduced to 1n. And if the frequency is very low, the capacitor should be increased to 1u or higher. 

In the diagram below a capacitor connects a device (sometimes called a TRANSDUCER) to a transistor amplifier. The capacitor is called a COUPLING DEVICE or COUPLING CAPACITOR as it couples the device to the amplifier. 
We have already mentioned how a capacitor works but in this situation an even simpler explanation exists:

THE COUPLING CAPACITOR

The capacitor works just like a magnetic window cleaner. A magnet on a rag on the outside of a window can be moved up and down by a magnet on the inside of the window.  
In our case, any voltage appearing on the left side of the capacitor "magically appears" on the right side. There are coupling losses but these are minimal. 

As mentioned above, some input devices need rail voltage applied to them (or a voltage lower than rail voltage) to produce an output waveform, others produce a waveform when they are struck or vibrated. 
The end result is the same - a WAVEFORM. 
But the problem is the SIZE OF THE  WAVEFORM. Some device produce a very large waveform while others produce a very small waveform. If the waveform is very small, a very high gain amplifier will be required. 

In the common-emitter amplifier we will be describing, the gain of a single stage is about 70 - 100. This may seem surprising because if you refer to data sheets for most small-signal transistors you find the DC gain is about 250 - 450. This is a DC current gain. We are talking about a voltage gain. When a transistor with a high current gain is placed in a common-emitter circuit, the automatic biasing provided by the base-bias resistor, produces a voltage gain of 70 - 100. We will discuss the reason for this in a moment but it is very technical. If you place the transistor in a pure DC circuit, you will achieve a current gain of up to 450. 

The fact is, you can only allow a voltage gain of 70 - 100 for the stages we are describing. That's not really a problem as one stage will provide 70x amplification and two stages will produce 70x70 = 4900x. We are talking about a voltage gain, so that if the "device" produces 2mV output, the amplitude of the signal after 2 stages will be nearly 10v. The only special value of voltage you need to remember is 5v for digital circuits (the old style TTL chips) and microcontroller projects. The voltage cannot be higher than 5v. For a HIGH to be registered on one of the input lines it must be about 70% of 5v = 3.5v.  
To achieve this you will require two stages of amplification if the "device" has a very low output. If the device has an output of more than 70mV, a single stage of amplification will be needed. 
There is no general rule for the size of the output voltage of a "pick-up device" however the table below is an approximate guide and shows how each device is connected. 

You will notice a capacitor has been added to the reed switch, ordinary switch and touch switch. These devices can be connected directly to the rest of the circuit however the capacitor has an inbuilt advantage. 
When it is included, the closing of the switch sends a pulse to the main circuit and if the switch remains closed, the detecting circuit can still be used to detect other switches. 

Some devices do not need a capacitor and sometimes a capacitor cannot be used. If the change in voltage is very SLOW, the increase or decrease will not pass through the capacitor. The reason why the signal passes through a capacitor is because the capacitor does not have time to charge and thus the signal on one side it passed to the other side.  This is why the voltage on one side can move up and down very quickly and the other side responds with an identical movement. 

Some devices do not need a capacitor because they have a high impedance and match the impedance of the amplifier PERFECTLY. An example is the piezo diaphragm. It has a very high resistance (infinite) and is effectively a piezo-electric capacitor. 
Another device not requiring an input capacitor is a set of water-level probes. If the amplifier is a digital gate, (such as a 74c14 Schmitt Trigger) a pull-up resistor will be needed. If the amplifier is a transistor, a pull-up resistor will not be needed (the base-bias resistor acts as a pull-up resistor). 

Now, the transistor amplifier:
There are a number of different common-emitter amplifier circuits, but the simplest is shown below. Other arrangements have better audio qualities but this is not necessary in our case. We require high amplification and any distortion produced by the stage is of no concern. The circuit consists of a load resistor and base-bias resistor. The input and output capacitors prevent any outside voltages influencing the voltage on the stage. This stage is called an AC amplifier as it only amplifies waveforms entering the stage via the input capacitor. 

THE SIMPLEST TRANSISTOR AMPLIFIER

The gain of the stage will depend on the rail voltage. At 3v, the gain will be about 50 - 70. At 12v the gain will be 100 or more. 
The entire stage consists of only three components (a transistor, and two resistors) and the resistors are chosen so that the voltage on the collector is HALF RAIL VOLTAGE. 
This allows the transistor to amplify both the positive and negative portions of the incoming waveform.
How the transistor sits with the collector at half rail is quite complex but it needs to be explained because the final collector voltage depends on the gain of the transistor as well as the value of the resistors. 
If you design a stage and the collector voltage is above or below half-rail, the stage will not produce the maximum gain. You must know how to bring a faulty stage to mid-rail operation. 
The design of the stage starts with the value of the LOAD resistor. Its value is selected so that 1/10th milliamp (100nanoamp) flows through the stage when it is fully and correctly but no signal is being processed. This is called the QUIESCENT current or IDLE current. It si also called WASTED current as it is the current that must flow all the time the circuit is turned on. 
For 3v rail, the load resistor is 10k. For 6v rail the load resistor is 22k and for 12v it is 47k.
Once the load resistor is selected, the base-bias resistor is selected so that the collector voltage sits at approx mid-rail. Experimentation has found this to be 1M for 3v, 2M2 for 6v and 4M7 for 12v. But these values correspond to a transistor with a DC gain of 450.
If you have designed a stage and the collector voltage is BELOW mid-rail, there are two things you can do:
1. Reduce the load resistor OR
2. Increase the base-bias resistor. 
If the collector voltage is ABOVE mid-rail, the opposite applies. 

HOW THE TRANSISTOR SITS AT MID-RAIL
This is the technical part. 
The transistor turns on and settles at mid-rail very quickly, so we will have to take it very slowly:
When the power is first turned on, the transistor is not conducting and the only things "in-circuit" are the load resistor and base-bias resistor. Current flows through these two resistors and turns on the transistor. The transistor is like a variable resistor and when it turns on its resistance decreases and forms a voltage divider with the load resistor. This means the base-bias resistor sees a lower voltage being supplied to it and thus a smaller current is passed to the transistor. 
The transistor keeps turning on but as it keeps turning on, the base-current being supplied to it decreases. BUT the transistor needs MORE current into the base to keep turning on HARDER and HARDER. 
So, an equilibrium point is very quickly reached where the transistor cannot turn on any more because the base-bias resistor cannot supply the required current. The result is the transistor turns on with the collector at half-rail potential.  This is exactly what we want and is due to the gain of the transistor, the value of the load resistor and the value of the base-bias resistor. 

CONNECTING TWO STAGES TOGETHER
We mentioned above,  two stages of amplification are needed if the "input device" has a very low output. 
The simplest arrangement is to connect two common-emitter stages together, separated with capacitors to keep the biasing of each stage intact. The diagram below shows a two-stage amplifier:

The two stage amplifier above is called a pre-amplifier,  high-gain amplifier or  2-stage AC amplifier. It will only amplify AC signals (waveforms).   
This amplifier will suit the following input devices:
1. A coil - simply connect the coil to the input capacitor. Suitable for frequencies above 100Hz.
2. A Photo transistor - suitable for frequencies above 100Hz. 
3. An Electret microphone - frequencies above 100Hz. 
4. A Piezo diaphragm - frequencies above 100Hz
5. A Light Dependent Resistor - suitable for frequencies above 100Hz. 

Input devices that can be connected directly to a Schmitt gate or microprocessor:
1. Reed Switch
2. Ordinary Switch
3. Hall effect device
4. Timing circuit

Input devices needing additional "special" amplifying stage(s):
1. Touch Switch
2. Water probes
3. Solar cell
4. Thermal sensor
Interfacing these devices will be covered in a future section. 

THE DC AMPLIFIER
Devices with an output voltage that changes over a long period of time require  a DC amplifier. 
DC amplifiers are critical and difficult to design because they do not contain capacitors to separate the stages. This means a slightly incorrect voltage on the input will cause the collector of the first transistor to be incorrect by approx 100x and the second stage will amplify this another 100x or more! The result will be a totally incorrect reading on the output.  A transistor in a DC circuit can exhibit a gain of 200x - 300x - or  400x. One of the biggest problems with a DC circuit is the alteration of the "bias-point" (the operating point for each of the transistors), as the temperature changes. Transistors contain P-N junctions and as the temperature changes, the voltage across the junction alters. This can change the current in the first transistor and the second transistor amplifies the change. The result is called an instability (thermal) problem. 
Forgetting thermal problems for the moment, the diagram below shows a typical DC amplifier. The first transistor is biased OFF due to the 2M2 and 220k base-bias resistors forming a voltage divider that puts 0.54v on the base.  This is below the "turn-on" voltage required for a transistor. The result is the first transistor is turned OFF and you can consider it to be "out-of-circuit."
It is easy to see the second transistor is turned on via the 10k resistor (on the base) and thus the output is LOW.
The circuit requires about 100mV rise on the input line for the first transistor to turn on. The voltage between collector and emitter drops to about 0.35v and this is below that required by the second transistor to keep it tunned on. The second transistor turns OFF and the output goes HIGH. 

The DC amplifier above requires a relatively high voltage to turn on. You may have only 1 or 2mV available (in the form of a rise and fall voltage) and require the amplifier to process the change. This requires a completely different circuit. 
The diagram below will amplify very small signals. A standard 8 ohm speaker can be used as a microphone and the circuit will provide a voltage gain of approx 1,000!

The main problem when designing a DC amplifier is the turn-on voltage of the first transistor. The base must see about 650 - 700mV for it to be in a state where it can be turned ON more (or less) when a slightly higher or lower voltage is delivered. 
This is a very difficult situation to achieve as the transistor is thermally sensitive, the components are thermally sensitive and the supply voltage must be absolutely stable. A slight change in circuit current due to any of  these components will be amplified by the transistor and also by any other transistors in the amplifier. A 2-transistor stage can have an amplification of 1,000 or more so the front-end must be extremely stable. 
The diagram below shows a DC amplifier that amplifies the voltage change across a diode. When a diode is heated, the voltage across it changes at a rate of approx 2mV per degree centigrade. This is amplified by the circuit and the result is shown on a 0 -1mA meter between the collectors of the two transistors connected in differential mode. This type of circuit has been selected to provide thermal compensation. Transistors contain P-N junctions just like diodes and if the diode is detecting temperature rise, the rest of the circuit may also be exposed to a temperature rise. By connecting the two transistors in differential-mode, any temperature rise detected by one transistor will cause the needle to move up scale. Any temperature rise detected by the other transistor will cause the needle to move down-scale. The result is the meter movement is cancelled out when both transistors see the same temperature rise. This leaves only the diode to influence the meter - very clever!

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