Hello,
I am trying to simulate a minimal NPN common emitter amplifier for 10 Mhz, using a BC547C.
Looking at the
datasheet it says:
Current − Gain − Bandwidth Product
(IC = 10 mA, VCE = 5.0 V, f = 100 MHz)
fT = 150MHz minimal, and 300MHz typical
I also drew the output characteristic curves using LTSpice (voltage supply 9V, Ib from 10µA to 250µA) with this circuit,
giving this plot:
From there I determined the active region to start at roughly Vce=440mV. So the output swing can go from 440mV to 9V without distortion, hence the operating point should be ideally at (9000mV-440mV) / 2 = 8560 mV / 2 = 4280 mV.
When chosing max(Ic)=45mA this gives me a resistor of 9V/45mA = 200 Ohm (not ideal but let's go with this for now). The range for Ib with the chosen DC load line is from about 10µA to 130µA, so the operating point for Ib = 70µA.
This results in the following circuit (10MHz current sine wave of 120µA peak to peak, with an offset of 70µA):
I intentionally used a current source and avoided input and output capacitors to be sure their added capacitance do not have any effect and used only one resistor Rc. So I suppose only the "built-in" capacitances of the NPN junctions affect the result.
As expected with kilohertz frequencies the result is a swing from almost 0V to 9V and as I go higher the swing reduces to 7V.
But for 1MHz and 10MHz there is a drastic change and barely any voltage at the output (about 300mV peak to peak).
The reduction seems much higher than expected given the mentioned GBWP from the datasheet.
Am I missing something?
Edit: Sorry it was unintentionally submitted before I was done writing the post.