Bredhurst Receiving and Transmitting Society

3. Technical Basics

3h.7 Recall and understand that a transistor (npn) can be used as an amplifier.

Note: the student is not required to recall transistor configurations. All circuits shown will be an npn transistor connected in common emitter mode.

Before we go into the syllabus fully we must take moment to describe what a transistor is and it basic operation.

The circuit diagram of an NPN transistor is:-

We previously discussed the PN junction when we where looking at diodes.

The transistor is called a Bipolar transistor it has 2 junctions and 3 separate connections.

The transistor has a thin layer of P type material sandwiched between 2 thicker N type layers hence the name NPN transistor or bipolar transistor.

The layer which forms the middle of the sandwich is called the BASE, the others are called EMITTER and COLLECTOR respectively.

The arrowhead points in the conventional direction of current flow.

The transistor like the diode need forward bias to turn the transistor on and then the main current will flow through the transistor from the collector to emitter .

NPN transistor used as an AUDIO amplifier

Transistor biased on by R1/R2. To compensate for changes in:

1. operating voltage (battery equipment, batteries getting tired)

2. variations in operating temperature

3. variations in transistor specifications (even from the same batch).

Then R4 is used to provide 100% negative feed feedback at DC. The reason this works, is because if the transistor takes more collector current, then this current passes through R4, and a bigger voltage would appear across it as a result. This voltage opposes the bias voltage that is set by R1/R2, and thereby stabilizes the transistor against changes as in 1) to 3) above.

Unfortunately, the inclusion of R4 also causes the transistor to have very little, if any gain (amplification) of input signals.

Fortunately, there is a simple answer, in that we can use a capacitor, (shown as C3, and known as the "emitter decoupling capacitor") connected across R4.

Inclusion of this capacitor allows SIGNAL voltages to go directly to the negative connection (earth), and NOT experience the 100% negative feedback that would otherwise be employed.

What we are doing here is using R4 and C3 to allow 100% negative feedback at DC, and no feedback at AC (signal voltages).

Negative feedback: Getting the output signal, and connecting it back to the input signal, in such a way that it opposes the input signal. The result is no amplification).

Positive feedback: This is something that we have all heard when a microphone gets too close to a loudspeaker. It "howls round". Useful in a circuit, when we want to make an oscillator!

Transistor amplifier circuits have similarities but different functions

You need to be aware that the simple amplifier circuit can have 4 basic variations BUT they are all based initially on the Audio amplifier.

	Audio amplifier This circuit is the Audio Amplifier distinguished by the resistor network around the transistor.
	Frequency Multiplier This is another form of amplifier and is a frequency multiplier distinguished from the circuit above by the use of the inductor and the tuning capacitor in the Collector.
	Low Power RF Amplifier This is another form of amplifier and is a low power RF Amplifier distinguished from the one above by the use of more inductors the tuning capacitors in the output of the Collector and the absence of resistors.
	RF HIGH POWER Amplifier This is another form of amplifier and is a RF POWER Amplifier distinguished from the one above by the use of transformers on the input and output.

3h.8 Understand that a small base current will control a larger collector current and that this is the basis of amplification.

As a result, it can be seen that a small, unintentional change in base current (current going into the base and out of the emitter) will result in a large unintentional change in collector current (current going into the collector and out of the emitter). R4 prevents this, whilst C3 allows a small SIGNAL current in the base to become a large SIGNAL current in the collector.

So, we have a transistor, acting as an audio amplifier. But what is so "magic" that is happening inside the transistor, to make the signal voltage so much bigger? Collector CURRENT changes are taking place inside the transistor.

Because this collector current flows through R3, then when little current flows, by ohms law, there is not much voltage dropped across R3. So, we have most of our 12 volts supply remaining. On the other hand, when the transistor is turned on harder, and takes more current, the voltage dropped across R3 increases, and as a result, the collector gets less voltage. If we take the output signal from the collector, we can see voltage fluctuations that are a replica, (but larger) than the input signal voltage.

Note that the output signal is an inverted (upside down) larger, replica of the original. Now, something that is true in all forms of life. YOU CAN'T GET SOMETHING FOR NOTHING!! What is actually happening is that the small signal you started with causes an output signal that is not "the original made bigger", but a larger copy of the original that is actually "made" of battery power.

Why is the signal "upside down"? When the input signal voltage rises, it turns the transistor on more. More collector current flows, and the collector voltage (which we are using as the output signal) falls. And vice versa...

This form of amplifier is called a "common emitter amplifier". This is because the emitter is connected to both the input and the output circuits. (One side of these are connected to earth, and, via C3, as far as SIGNALS are concerned, the emitter is connected to earth. Other forms of amplifier may, or may not, "turn the output signal upside down".

3h.9 Understand that if the base current is large enough, the transistor behaves as a switch such that the current in the collector can be turned on and off.

NPN transistor used as a switch

Here, we are not concerned with amplification of a signal such that we want a faithful (and larger) reproduction of the signal on the output of the transistor. We need DC stability (i.e. the transistor should stay "off" when intended, and not be affected by transistor type variations, temperature, or supply voltage). We also need the transistor to turn fully "ON", again not affected by the above variations.

This is fortunately simple; we have only to ensure that there is a resistor connected between base and emitter (R3) to ensure it is held "OFF", and then apply, via a resistor (R1), some current to the base. What we apply needs to be enough to ensure that the transistor is fully turned on despite variations as above, and also be enough to allow some to go through R3, (This resistor "nicks" it from the transistor!).

The on and off concept according to the configuration.

Ask your tutor to build a small circuit based upon the above so that you can see the difference between the two circuits in that sometime applying a current to the base turns the led on and sometimes it is the other way round that applying a current turns the led off.

Whilst not part of the syllabus you might like to know that the other type of transistor is a PNP transistor has a thin layer of N type material sandwiched between 2 thicker P type Layers. The arrowhead on the emitter symbol distinguishes the transistor as being either NPN or PNP.

3h.10 Recall that a transistor can be used as an oscillator to generate audio and radio frequencies by maintaining the oscillations in a tuned or frequency selective circuit.

A simple fact to be recalled is that a transistor can be used as an oscillator to generate audio and radio frequencies.

The sort of question you might be asked in this section is what can a transistor be used for of which 3 will be spoof answers.

3h.11 Distinguish between a crystal oscillator and a variable frequency oscillator (VFO) based on a tuned circuit.

Diagrams will show the Colpitts oscillator with the transistor in emitter follower mode. Students are not expected to recognise different types of oscillator.

The above are not practical circuits as no off bias exists to the transistor BUT the important parts to recognise are those that have been highlighted.

The diagram fig.1 on the left shows a crystal and thus is a fixed frequency Crystal Oscillator. Do not be confused with the variable capacitor into thinking that it is a variable oscillator as the variable capacitor is there to bring the crystal oscillator to resonance at a particular frequency. sometime the variable capacitor is not even in the circuit.

The diagram fig.2 on the right shows a tuned circuit highlighted and also shows a variable capacitor in parallel with an inductor, making up the tuned circuit, and thus is a variable frequency oscillator.

A radio frequency oscillator consists of an amplifier, with POSITIVE feedback that is at one particular frequency. The component that determines the frequency is either a tuned circuit (a coil, and a capacitor), or a crystal. The coil and capacitor (L1 and C1 in Fig 2) have a pleasant habit of being able to exchange energy between them at a particular rate. (The desired frequency). They are "a bit lossy" at other frequencies.

Diagram opposite shows the circuit diagram of a frequency modulated Variable Frequency Oscillator (VFO). The tuned circuit formed by L1 and VC1 and the additional capacitance presented by C1,C2 and C3 determines the frequency of oscillation.

Typically this would be in the order of 6 to 12MHz. The diode D1 is reverse biased by the voltage set at the junction of R4 and R5 and applied to the diode through the DC path via R6 and the Radio Frequency Choke RFC1. The reverse DC bias voltage should be chosen so the diode is operating at a point where the applied audio modulating signal results in a linear change in capacitance. (i.e. Equal positive or negative excursions of the amplitude of the audio frequency (AF) modulating signal give equal decreases or increases in capacitance). Commercially available diodes provide relatively large changes of capacitance for a given change in applied voltage. If this VFO was operating at 12MHz and was going to be used on the 2M band at 144 MHz then the frequency multiplier stages would have a multiplier of 12 (12 X12 =144). This multiplier of 12 would also be applied to the frequency deviation produced by any frequency modulator. This means that a deviation of 2.4kHz at 144MHz is produced by only a deviation of 200Hz at the 12MHz operating frequency of the VFO. To reduce the capacitance change presented to the VFO tuned circuit C10 is connected in parallel with the modulating diode D1 and the low value capacitor C9 couples the modulator to the VFO tuned circuit. Typically with this configuration and component values shown an AF signal of 1 volt will produce a deviation of 200Hz of the 12MHz VFO. It should also be noted that the VFO and modulator should be supplied from a well regulated +12V DC supply as any change in DC operating conditions will result in a change of the reverse bias to D1. This will consequently cause a change in capacitance of D1 and a corresponding shift in the frequency of the VFO. RFC1 presents a high impedance to the 12MHz RF present across D1 ( it presents a relatively low impedance to the AF signal) and in combination with C8 acts as a low pass filter minimizing RF entering the AF stages. R6 in combination with C8 constitutes a low pass filter and attenuates higher AF frequencies.

Lossy: You put some in, and don't get it all back out! The transistor amplifier only has enough gain to make the circuit oscillate when the coil and capacitor are not "lossy" (i.e resonant), it oscillates at one frequency.

Gain: The ability of the circuit to take a small signal, and make it bigger).

The advantage of using a coil and a capacitor to determine the required frequency is that either component may be varied to alter the frequency. It is not difficult to alter these components, and variable types are available. The down side of doing so is that a number of things (like a change in temperature) can cause the actual value of the coil, or the capacitor to vary, and as a result, the frequency changes.

Using a crystal to determine the frequency as in fig 1 (X1) will result in a very stable oscillator, but it is very difficult to alter the frequency, even by a very small amount. Some years back, in order to have a stable frequency in VHF radios, crystals were used in both the transmitter and receiver to determine the operating frequency. Unfortunately, to change channel, you switched in a different pair of crystals. This was expensive, (£10 a channel or so!). We didn't have too many channels available in those days!

Frequency of a crystal: Think of a crystal as being similar in a way to when you sing in the bath! You will notice that certain notes you sing are a lot louder than others, and in fact, at audio frequencies, the bathroom (being generally a small, unfurnished room) resonates at particular frequencies. Just as it is not easy to alter that by changing the size of the room, you would need to physically grind the crystal to make it thinner to alter the frequency it works on (very hard, but it can be done, and the frequency would increase), or make it bigger to get the frequency down (impossible!).