Bruce CarpenterScience Coordinator,Bendigo Senior Secondary Collegecarpenter.bruce@bssc.edu.au

The transistor turns 60!Invented at Bell labs USA December 1947.

About the size of a modern mobile phone.

We can now fit 2 million transistors on a full stop. .

Timeline:

1947 Transistor invented.

1948 Shockley develops first semiconductor transistor

1954 Texas Instruments introduces transistor radio.

1965 Intel co-founder Gordon Moore coins Moore’s Law.

1981 IBM launches the PC

2007 Intel demonstrates chip with 1.9 billion transistors.

Integral to computers, CD/DVD players, iPods, mobile phones, digital cameras, video recorders, Plasma and LCD TVs, in fact almost every modern electronic device imaginable!

Trans – “across”

…istor –”resistor”

• The transistor can be thought of as a device whose resistance (across collector-emitter) varies with input current (at the base).

A water pipe analogy

• The collector (C) collects water at the top, the emitter (E) emits water at the bottom, and the flow of water current is controlled by a small tap and valve, similar to the base lead (B) of a transistor.

Water Pipe model (cont)

• When the tap is jammed on (valve vertical), maximum water flows. Analogous to transistor saturation. IC = IE at all times.

• When the tap is jammed off (valve horizontal), no current flows. Analogous to transistor cut off.

• These two states are used in all digital circuitry using transistors. Only ON or OFF (1-0) states are possible.

Analogue Transistor Action

• If we turn the tap half on and then continuously twist the tap clockwise and anticlockwise, then the change in water current flowing from collector to emitter will be proportional to the twisting action.

• This is analogous to a transistor operating in the linear region. Small changes in the base current cause much larger but directly proportional changes in the current flowing from collector to emitter.

• The transistor can be biased using a DC power supply and resistors to act as an amplifier.

• Small AC signals are applied via the input capacitor, which allows AC but not DC to flow through it.

PART A: TRANSISTOR BIASING

Q1. Use a digital multimeter set to read DC Volts to measure the supply voltage, the voltage across each of the four resistors, and the transistor voltages VBE, and VCE.

Vsupply V10K

(bottom)V3.3

K(top) V3.3K V1 K VBE VCE.

9.18v 6.90v 2.27v 4.40v 1.33v 0.66v 3.42v

This is what it looks like on breadboard

Q1a) Use Ohm’s Law to calculate IC and IE (in mA).

  IC = VRC/RC = 4.4/3.3k = 1.33mA

IE = VRE/RE = 1.33/1.0k = 1.33mA

 Q1b) How do they compare? Explain your answer.

  They are the same because virtually all current flows from collector to emitter. The base current is very small as IB = IC/

= Transistor current gain = 100 to 800 for a BC108 transistor.

Q1c) Explain the purpose of the voltage divider (combination of 10 K and bottom 3.3 K resistors). Use your measured voltages to explain your answer.

To provide sufficient quiescent DC operating conditions, small DC base current and correct biasing of the base-emitter (pn) junction of the transistor.

Note V10k + V3.3K = 9.17volt = Supply voltage

PART B: TRANSISTOR AMPLIFIER

Q2. These DC voltages levels bias the transistor so that it will operate correctly as an amplifier.

Apply a small AC voltage signal of frequency 1000Hz from a signal generator to the input of the amplifier and observe input and output signals on the Cathode Ray Oscilloscope.

2a&b) Measure and record the peak to peak input & output signals. (Vertical gain is set to 1 volt/div)

Vpp for input signal = 1.6 volt

(bottom trace)

Vpp for output signal = 5.0 volt

(top trace)

Note how the traces indicate that this is an inverting amplifier

2c) Hence calculate and record the Voltage Gain of this amplifier.

Voltage gain = Vout/Vin = 5.0/1.6 = 3.1

Note that this is approximately the same as the “rule of thumb” for voltage gain,

Av = RC/RE = 3.3k/1k = 3.3

2d) Explain the purpose of the input and output capacitors.

  The purpose is to block any DC voltage from an input transducer such as a microphone which could alter the DC biasing conditions for the amplifier to operate correctly.

Similar for output.

2e) Increase the input voltage until “clipping” occurs. Explain why this happens.

Clipping occurs when Vin peak to peak is approximately 1.8 volt.

The output signal can no longer follow the input signal because the amplifier has moved out of the linear region.

More severe clipping occurs as the input signal increases further

2f) What would be the effect of clipping if his amplifier was used to amplify music signals (say from a microphone). Any musical signals would sound

distorted.

Further hints

• Use a 9 volt battery and battery snap for the DC power supply.

• This can avoid earthing problems when you attach the CRO and signal generator to your amplifier.

• Use a cheap multimeter to measure DC voltages. This one costs approx $10.

• The (beta) or current gain of a transistor can be measured using a digital multimeter.

• The BC108 transistor used in our amplifier is npn.

• Use red and black alligator clips to connect CRO and signal generator to components on breadboard.

• Use black for 0 volts DC from CRO and Sig Gen.

Hoping you enjoy the transistor topic!