The care and feeding of our friend the transistor is an art; many spend their lives learning it and all that is entailed. In yesterday's post I showed a cascode circuit consisting of two such devices needing to be fed. This subject came up when those watching our new video series on the BHK amplifier heard mention from Bascom. Q1 is easily fed by a connection to the music source, which could be a preamplifier if we were discussing the input of a power amp. Q2 is much different, connected only by Q1 in a technique known as Common Base. Amplification devices have three terminals to them: Emitter, Collector and Base if it is a transistor. Here is a picture of how one is represented. Typical connection for this amplifying device would be to place the music into the Base (B). If you wanted to make an amplifier with power to drive a speaker, your output would come from the Emitter (E) and it would be the same size as what you put into the base. If, instead, you wished to make the output larger than the input, as you might in a preamplifier, you use the Collector as your output (C). This classic circuit description is how we configure tubes and transistors of all types and if you look at the original drawing, Q1 is setup to make the signal larger, as in a preamplifier. Using this configuration we have many fine qualities: high input impedance, reasonable distortion and linearity among them. If we change the configuration, however, so the music goes into the Emitter first, rather than the base, (as we see in Q2) our amplifying device takes on a new set of characteristics better than the classic just described: improved linearity, distortion, and faster transient speed. So why not always use that which is represented by Q2? Input impedance is low, nearly zero and connecting a preamplifier to this point would squash its output signal rendering it useless. It needs something in between, which is Q1. Together, the two transistors give performance levels identical to a single device, which is why we use it. Lastly, you might wonder if the grounded base style of connection is useful on its own, given its very low input impedance and the answer is yes. A moving coil phono cartridge loves to see a very low input impedance and this is a perfect place for just such a circuit without benefit of Q1.
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