Single Transistor Amplifier Revisited – Part 1

Published:2013/10/11 20:51:00 Author:lynne | Keyword: Single Transistor Amplifier Revisited – Part 1 | From:SeekIC

Circuit and Biasing Technique There is so much to learn about the single-transistor amplifier, that this brief tutorial hardly scratches the surface. This discussion considers only the common-emitter configuration as applied to low level audio. History In the early days of solid state amplifiers, thermal stability was the big issue. The first devices available were leaky germanium PNP transistors. The collector to base leakage was often so excessive that it could cause thermal run-away because the leakage increased exponentially with temperature. The classic way of keeping this under control was the base divider-emitter swamping resistor topology. Early text books (including the one I used in 1963) had a detailed section on this and included a mathematical calculation for “stability factor.” Unfortunately, now (some 50 years later), we are still suffering from vestiges of this approach as we continue to see the same circuits popping up even though germanium transistors have been obsolete and unavailable for well over 30years, and the silicon bipolar NPN has been long the transistor of choice. Since leakage in silicon devices is so low that it can hardly be measured, we can make a fresh start. Self-Biased Circuit Schematic A stable quiescent operating point (“Q” point) can be established simply by sourcing the base divider from the collector voltage. This dispenses with the emitter swamping resistor. While not perfect, it provides predictable results and simplicity. It is good for low power amplifier transistors that dissipate less than about 100mW. R1, 2 & 3 form the base divider. The juncture of R2 & 3 is bypassed to common via C2 to eliminate negative feedback from the collector—this negative feedback tends to reduce voltage gain. We will be covering negative feedback in the future. C1 is the input coupling capacitor and C3 is the output coupling capacitor—both pass the AC signal while blocking the DC component. To accommodate a wide range of hFE’s, the base divider current is in the range of 5 to 10 * base current. Operating point calculations (ohms law) Set collector voltage: My rule-of-thumb is to set it at about 40% of Vcc. In this case it is 5V. Calculate collector current: Ic = (Vcc – Vc) /R4 = (12V – 5V) /2.2K = 3.2mA. Calculate base current: Ib = Ic / hFE = 3.2mA /200 = 16uA (using the common 2N3904) Establish base divider current: Id = Ib * 5 = 16uA * 5 = 80uA (a factor of 5 is good) Calculate Ir1: Ir1 = Id – Ib = 80uA – 16uA = 64uA Calculate R1: R1 = Vbe / Ir1 = 0.65V /64uA = 10K Calculate R2 + R3: R23 = (Vc – Vbe) /Id = (5 – 0.65V) /80uA = 54K Calculate R2,3: R2 = R3 = 54K / 2 = 27K (may be unequal, but total must be 54K)



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