Hello,
I designed an amplifier with 10x gain, built it on a breadboard and the gain was almost 10x. Then I simulated the same circuit and the gain is only 9x.
The amplification should be given by resistors R3 (560) and R4 (56).
I tried the model of transistor from the manufacturer’s website and the default NPN kicad spice model, no difference.
Have you any idea what I did wrong?
Thank you!
A_amplifier.rar (42.2 KB)
The emitter and collector resistor values look very low for a BC546. The internal Re of the transistor is in series with R4 and reduces the gain slightly. I am not sure how the simulation models this.
I found an article that says what you said, there is a small internal resistance that is in series with the RE. So now I increased values of collector resistance and the emitter resistance, ratio is same, but the result is still the same, gain only 9x. In addition, I wouldn’t help myself if the measurement turned out correctly, the tolerance of the resistor values should be 1%.
LTSpice gives a gain of x 9.4.
The Re = 25/I(ma) resistance fundamental to BJTs increases with your resistance values
Have you taken into account the tolerances of resistors? What about the temperature of the transistor’s junction? - that internal emitter resistance is also just a mathematical model, even if it’s meant for pen and paper calculations, and it’s highly temperature dependent. SPICE by default simulates semiconductors at a junction temperature of 25 degrees Celsius - in real life this might not be the case, especially since the transistor will heat itself up when biased.
Overall achieving about 10% difference between simulation and reality for discrete transistor circuits is not actually that bad in my opinion.
Tolerances of resistors is 1%. I dont know influence of temperature, but i dont have probleam with real measurment. The results of the theory were consistent with the actual measurements. I have a problem with the results based on theory not matching the simulations. I do wonder about 10% difference between simulation and theory.
I am also confused by the difference.
But also:
2080/216 = 9.62962962962963
Which makes the difference:
1 - 10/_ = -0.03846153846153855
So about 4 percent.
Next, this makes me wonder about the accuracy of voltage measurement on your oscilloscope. But still I would expect the amplification closer to 10 then to 9.
Ok, so I reduced RE and RC, ration is still 10, but 9x is still my best result. maybe it is just the combination of calculation errors and measurement deviations that the measurement has the same result as the calculation.
Based on this formula, gain is 9,1x
perhaps my calculations are too simplified and the oscilloscope too inaccurate
OK, so I took a too simplified approach to the calculations and just took the ratio of the resistors into account. It’s been a long time since I did calculations like this.
So after all it looks like everything is within normal tolerances. Do you have any data on the accuracy of your oscilloscope measurements? These things generally only have an 8 bit ADC, and when the signal amplitude is half the screen, only 7 bits are left, which is not much.
My old Rigol DS1052-E is atrocious in this regard. It’s one of the reasons I bought an Siglent DSD1104X-E recently, but I have not yet made much use of it yet.
It seems everyone is assuming the sample set of ONE physical transistor is fully representative of the type.
Every part in your system will have a tolerance range. Testing each component with well calibrated equipment might give a better result? I remember a video of Dave on EEVBlog having a big fail because he “ass u me’d” that he could buy a lower bin set of devices and their tolerance would be closer to nominal than fringe. It appears the batch as a ‘near miss’ but good enough to sell to the unsuspecting public.
I should have 8bit scope.
I’m glad I have an oscilloscope at all. To solve the problem with “well calibrated equipment” is out of my league.
But I would like to focus on calculation and simulation. And it seems yellow text is not true, based on the previous discussion.
This one is better
unfortunately I can’t find r´e in datasheet, it is supposed to be 25mV, for small signal bipolar transistor
r’e depends on the collector current and temperature. A good approximation is 1/gm, where gm, the transconductance is Ic/VT. That VT in mV is 0,08625 * TJ in Kelvin, which in room temperature is around 25-26mV. Keep in mind that all of the above are approximations - so slight differences between simulation, manual calculation and reality are to be fully expected.
Also if you need precise gain, a precision operational amplifier is the way to go.
