Hello all,
I am currently trying to give current gain to a signal which has spectral content primarily at 40KHz and about 167Hz. (The waveform is a series of 1ms pulses of 40KHz square wave spaced apart by 5ms).
I am trying to use a common emitter transistor amplifier for this, which is shown in this diagram i've attached.
Using a oscilloscope, it seems to work ok in amplifying the signal except the 1ms pulses almost completely their 40kHz content, and just become 1ms pulses of logic high voltage.
I tried checking the datasheet of the transistor (CDIL BC108), and the transitional frequency is given as 150MHz.
I then measured the voltage at my Rb, and Rc, and using V=IR, found out the Ib was 3.7μA, and the Ic was 0.26mA, so the gain should be about 59.
According to these measurements then, and the equation Transitional frequency = Gain x frequency at which gain is at -3dB of low f gain,
150 000 000 / 59,
the -dB frequency should be 2.5MHz, and the transistor should have no problem in amplifying 40kHz content.
Following this I measured the frequency response of the transistor myself, and found out amplification dropped to nothing after just after 10kHz.
Can anyone tell me why my findings are diffrerent to the datasheet, where and how I'm going wrong with my understanding? Thanks in advanced!!!
Jonathan
I am currently trying to give current gain to a signal which has spectral content primarily at 40KHz and about 167Hz. (The waveform is a series of 1ms pulses of 40KHz square wave spaced apart by 5ms).
I am trying to use a common emitter transistor amplifier for this, which is shown in this diagram i've attached.
Using a oscilloscope, it seems to work ok in amplifying the signal except the 1ms pulses almost completely their 40kHz content, and just become 1ms pulses of logic high voltage.
I tried checking the datasheet of the transistor (CDIL BC108), and the transitional frequency is given as 150MHz.
I then measured the voltage at my Rb, and Rc, and using V=IR, found out the Ib was 3.7μA, and the Ic was 0.26mA, so the gain should be about 59.
According to these measurements then, and the equation Transitional frequency = Gain x frequency at which gain is at -3dB of low f gain,
150 000 000 / 59,
the -dB frequency should be 2.5MHz, and the transistor should have no problem in amplifying 40kHz content.
Following this I measured the frequency response of the transistor myself, and found out amplification dropped to nothing after just after 10kHz.
Can anyone tell me why my findings are diffrerent to the datasheet, where and how I'm going wrong with my understanding? Thanks in advanced!!!
Jonathan
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