BJT common base with inductive source = higher frequencies gone - Page 1
Forum section: electronics / projects

author: dusan
author_rank: Regular Contributor
timestamp: November 02, 2021, 05:15:36 pm
content: I'm designing single stage BJT preamp for inductive transducer (20ohm 20mH 5mV peak-peak voltage). The load is 1700ohm (soundcard). I stared with common emitter but because load is quite low, I had to crank amplification way up (~75x without load). It works but this brings 2 problems:- it's very sensitive to exact resistor values, 1% change and gain quickly goes down- it's very sensitive to supply voltage, 1V drop from 9V to 8V will cause 40% gain lossSo I thought, common base. I designed it in simulator everything worked perfectly. Both two problems of common emitter were solved. But then I found out higher frequencies are completely gone in common base version. I added correct inductance to source in simulator and sure enough, the AC analysis of common base drops basically to zero at 5kHz, while common emitter is practically unaffected by inductive source.Can this be fixed somehow in common base?

author: magic
author_rank: Super Contributor
timestamp: November 02, 2021, 05:29:13 pm
content: Common base has no current gain. An AC current equal to load current has to flow through that poor transducer.The solution for biasing a discrete CE stage is to set the base a few volts above ground, choose the emitter resistor to provide the desired emitter bias current and then parallel it with a capacitor. Or a capacitor and a small resistor in series to improve linearity.

author: duak
author_rank: Super Contributor
timestamp: November 03, 2021, 01:31:47 am
content: Dusan, do you know what the term "input impedance" is and how to calculate it for your circuits? If not, it is the equivalent RLC that the amplifier circuit appears as to the transducer.The common emitter amplifier will have an impedance on the order of 5K ohms. Since the transducer inductance is in series with the transducer voltage source it will form a low pass filter with a corner frequency of approximately 630 Hz.The common base amplifier will have a much lower impedance on the order of 50 to 100 ohms. This forms a low pass filter with a corner frequency of 6.3 to 12.6 Hz. Not only that, the voltage gain is a function of the ratio between the impedances seen at the collector divided by the impedance seen at the emitter, as well as the intrinsic properties of the transistor. As the frequency is increased, the reactance of the transducer increases and reduces the voltage gain.To increase the bandwidth of the amplifier you will have to increase its input impedance. One way is to use a common emitter configuration, which can have an input impedance of 50K to 100K, but it will have a voltage gain of just below 1. If you need a greater gain you will have to add another transistor stage to amplify it further. There are a number of circuits that can do this and I've attached one that I found on the 'net.As a starting point I would increase the values of the 22K, 5K and leftmost 100 ohm resistor each by a factor of 10. If you need a greater voltage gain you can increase the value of the 1K ohm resistor on the collector of Q2. You may have to vary the value of the 22k resistor to set the DC voltage on the collector of Q2 at about 60% of VCC.