Build your 1st op amp with the SAKit!
The Simple Amplifier Kit (hereafter SAKit) is ideal for engineers, hobbyists & technicians who want to know analog circuits @ the transistor level before delving into analog ICs.
SAKit is a VFB (Voltage Feedback) operational amplifier designed to make use of five transistors with class AB output stage hookup as an AC coupled non-inverting amplifier. It is the embodiment of a simple classical op amp (a class A output stage will reduce transistor count to four at the expense of lower current sinking capability & efficiency). Although this circuit is quite simple (some might say trivial!!) because there are no emitter constant current source, collector current mirror, output current limiting, etc. it serves to illustrate the starting point in op amp design. Once the kit is built on a breadboard you have a fully functional, one-channel audio amplifier, complete with 2 inch 8 ohm speaker & 3.5 mm phone plug to verify audio performance (subjectively) using an ordinary mobile phone with mp3 player. A 3.5 mm phone jack is provided to listen via headphones. The 3.5 mm jack & plug, speaker & 5 ohms resistor must be soldered to AWG 22 solid wire before inserting into the breadboard.
2 BC546 transistors (Q1 & Q2) with resistors R2 & R3 forms the differential input stage, a 2N3906 transistor with capacitor C3 & resistor R6 as voltage gain stage (aka VAS). Output stage consists of TIP31 Q4 & TIP32 Q5 transistors, diodes D1 to D3 & resistors R7 & R8. Input signal is applied to capacitor C1. Feedback network is composed of C2, R4 & R5; short C1 & C2 to amplify DC signal. Capacitors C8 & C9 prevent DC voltage reaching the speaker. R9 is optional to protect the output stage from short circuit. Recommended power supply is +-15V.
The kit comes with an adjustable dual-tracking reference as DC input voltage to the amplifier and a μA741 (maybe the most popular) op amp for operation comparison.
Details of op amp circuit analysis & operation can be found at
2. The Art of Electronics 2nd Edition pages 236 to 238, Paul Horowitz & Winfield Hill, Cambridge University Press 1980, 1989, ISBN 0-521-37095-7