A transistor in a circuit

(a case study)

With the help of some slides (1 to 10) you can understand how you can transform a transistor in an amplifier circuit. You have to 'find' 4 resistors, connect them to the collector, the emittor and the basis, and calculate there values.

Slide 1. If you have a transitor he is doing nothing without an electric current.

Slide 2. For a current you need a voltage, so take a battery. Here: +12 Volt (DC).

Slide 3. This transistors likes to have 1 milliAmpere (mA). How can you arange 1 mA?

Slide 4. Take two resistors. Make a selection in their resistance value, until the current is 1 mA.

Slide 5. In this case (here) resistor 1 is 4 kOhm ('RC') and resistor 2 is 1.99 kOhm ('RE') and so the transistor voltage is become between +7.5 [V] (DC) (on the collector c) and +2.2 [V] (DC) (on the emittor e).

Slide 6. In a transistor there is always a difference in DC voltage between the basis (b) and the emittor (e) of +0.7 Volt. Without this 0.7 Volt (and the very very small current through the basis) the transistor doesn't work. Here 2.9 [V]. How you can arrange a current and a DC voltage between b and e?

Slide 7. Create a voltage of 2.9 [V] with two resistors: R1 and R2. The current is as small as possible. Here R1 is 174 kOhm and R2 is 56 kOhm. Now the transistor is prepared for DC. The currents and voltages didn't change (in time). The currents and the voltages are stable: DC. To transform a transistor circuit to an amplifier (for AC) you need a AC source.

Slide 8. If you have a AC voltage source (ui), this transistor circuit can amplify this (to uo). The amplifier coefficient (uo/ui) depends on the 4 resistances RC, RE, R1 and R1; not the transistor itself.

Slide 9. Now you can use this transistor circuit for amplify signals.Amplifing voltage or current signals (AC signals) is only possible if there are DC voltages (and/or DC currents). See the analogon with waves in a bathroom (Min, 2001; online). "Without water: no waves". "Without deep water: no big waves".

Slide 10. The DC levels (2.9 and 7.5) on the basis and collector is a condition for succes in amplifing signals. If there is a AC signal the main voltages are always 2.9 and 7.5 [Volt], the DC values. Without sufficient DC levels, big signals (bigger then the drawn 6 and 9 Volt output signal), are not possible. The minimum of the output is 0 Volt; the maximum is 12 Volt.

Schaum (19xx) McCraw-Hill; partly online.
Min (2003) Transistor; online.

Enschede, June 2004