This is the setup for C1815 npn transistor:

Input wave form:

Wave form at base:

Wave form at collector:

The C1815 transition freq is 80 MHz, so it should work at 1.7 MHz right ? Why can't it turn on and off at all ? It looks like it always conduct. Is it because of input capacitance ?

 

This is the setup for C1815 npn transistor:

Input wave form:

Wave form at base:

Wave form at collector:

The C1815 transition freq is 80 MHz, so it should work at 1.7 MHz right ? Why can't it turn on and off at all ? It looks like it always conduct. Is it because of input capacitance ?

Hi Dangky, the information you posted is a bit limited. Are your power supply negative, the transistor emitter, the signal source common and the scope common all connected together? If not, that will cause your problem.

 

Hi Dangky, the information you posted is a bit limited. Are your power supply negative, the transistor emitter, the signal source common and the scope common all connected together? If not, that will cause your problem.

I'm sure I connected the power supply correctly. And they have a common ground of course.

 

Further, you show an input waveform of about 3V peak to peak, but what is the actual voltage level at the bottom (or top) of that waveform?

 

Further, you show an input waveform of about 3V peak to peak, but what is the actual voltage level at the bottom (or top) of that waveform?

Do you see the [1] pointer on the left ? That is scope ground, connected to the common ground.

 

Those voltages don't make sense.
It is what you would see if the 5V was connected to the resistor on the base and, maybe, the input was connected to the collector.
Double check the transistor type, and its connections according to the datasheet, and the connections to power supply and input signal.

 

Why can't it turn on and off at all ?

Apply 5V to the base resistor with the emitter grounded and the collector connected to 5V with the 1k resistor. Report the collector voltage.

Then ground the base resistor and report the collector voltage.

 

Is this how you connected your circuit and took your measurements?
What's the nature of the input signal? Are you intending to amplify the signal or clean up the spites?


Allen

 

Those voltages don't make sense.
It is what you would see if the 5V was connected to the resistor on the base and, maybe, the input was connected to the collector.
Double check the transistor type, and its connections according to the datasheet, and the connections to power supply and input signal.

Yeah it doesn't make sense, that's why I'm asking. I double checked everything.

Is this how you connected your circuit and took your measurements?
What's the nature of the input signal? Are you intending to amplify the signal or clean up the spites?

Yes the diagram is correct. I want to amplify it (npn as a switch).

I divided the frequency by 16 and checked again. Here is the result. Note that my scope has only 1 channel, so wave forms aren't captured at the same time, but aligned by triggered time. From top to bottom is collector, base, input voltage.

You can see that when input voltage came down to zero, base voltage stayed at 0.7v for some time. So with higher frequency, base voltage is always at 0.7v is understandable.
What I don't understand here is why base voltage took such long time to go down.
Can anyone with a oscilloscope do the same experiment and verify it for me please ? Maybe my scope or npn transistor is bad quality ?

 

Well I did some search and saw the baker clamp, and why I need it. So to increase the working frequency I need more part.
I will look into fet transistor to see if the diagram is more simple.

 

Is this how you connected your circuit and took your measurements?
What's the nature of the input signal? Are you intending to amplify the signal or clean up the spites?

View attachment 138613
Allen

Do you want to try the circuit in breadboard, I'm afraid of the R1, R2 are too high when they are working over 1 Mhz?

 

Here's the sim in proteus. With 10K resistors the output was badly distorted becoming sawtooth wave.

I am not able to sim 1.6MHz with analog so I use digital clock input but has to bring it down to 3V using a voltage divider. Not sure if 2SC1318 is better than BC108 or not....

Allen