THE
COMMON BASE AMPLIFIER See this article and return This circuit
converts an ordinary speaker into a very sensitive microphone. The collector current is almost the same as the emitter-current and now the additional 0.125mA will REDUCE the collector current and the change across the 2k2 will be 2,200/8 = 275mV. Thus the collector voltage will RISE 275mV. Thus the gain of the circuit is 275. We are already assuming the voltage on the collector is 3v and the 47k has been selected to create this 3v. Taking the circuit as a whole, the collector can increase by 2.75v and decrease by 2.75 before clipping occurs (because we have 3v available) and thus the speaker can produce a 20mV p-p before clipping. (because 20mV will produce a voltage of 275mV x 20 = 5.5v on the collector = 2.75v increase and 2.75v decrease.) And the gain of the transistor does not come into the calculation. The cone will produce a waveform in the opposite direction during the second half of the cycle and the collector will FALL 275mV or 2.75v according to the mV produced by the speaker. No-one has ever described this critical feature and unless you know what is occurring, you cannot interpret a trace on a DIGITAL oscilloscope. Common Base and Common Emitter stages directly coupled together This circuit adds a Common Emitter stage to the Common Base to produce a DC coupled (Directly Coupled) amplifier with very high gain. The common-emitter transistor can be called a BUFFER stage as it provides a lower impedance output than the first stage. THE BOOTSTRAP CIRCUIT Another very interesting circuit is the Bootstrap Circuit. The output of the second transistor has been taken back to its input to produce an improvement called a BOOTSTRAP Circuit to create an even higher gain. It uses positive feedback to achieve this very high gain. The two transistor circuit shown above has a gain of approx 1,000 and converts the very low output of the speaker into a waveform that can be fed into an amplifier. The circuit is simply a common-base stage and an emitter-follower stage. But the output of the emitter-follower is taken back to the input of the same stage and this is the Bootstrap feature. It is like pulling yourself UP by pulling your shoe laces. When the voltage from the speaker reduces by 1mV, the transistor turns ON a little more and pulls the collector voltage lower. This action takes a lot of effort and to pull it lower, requires more energy from the speaker. In the Bootstrap circuit, the common-base transistor pulls the 10k down and this pulls the emitter-follower transistor down. At the same time the upper 22u is pulled down and it pulls the 10k down to assist the first transistor. In other words the first transistor finds it much easier to pull the 10k resistor down. When the common-base transistor turns off, the 2k2 pulls the 10k resistor UP and it is aided by the top 22u. The end result is a very high output voltage swing. The loud speaker will detect a pin drop and it will become a very sensitive microphone. You can use the circuit for a low impedance transducer such as a coil. These are normally very difficult to interface.
Page-1 Common-Emitter stage
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19/2/2024 |