Logic level converter with Mosfet

Thread Starter

gjsm

Joined Nov 3, 2023
4
Hello,
I want to convert a PWM signal in which the high signal is in the range of 2.2V to 3.3V to a PWM signal with a 3.3V high level.
Additionally, I want an LED to blink when a signal is present at the input. I have designed a simple circuit for this using a MOSFET.
I plan to tap the output voltage through a very large resistor to obtain a non-inverting output signal. I chose the BS170 MOSFET for this, as it fully switches at a threshold voltage of 2V. The circuit works well at frequencies below 50kHz, but I need to be able to convert frequencies of at least 100kHz.
At these frequencies, the MOSFET no longer switches off quickly enough, resulting in distortion as shown in the attached image.
The MOSFET has an fT of 1MHz, and the turn-on and turn-off time is typically 4ns and a maximum of 10ns. These values should not be a problem at 100kHz, so I am puzzled as to why I am experiencing such distortion at high frequencies.

Does anyone have an idea what could be causing this?

* In LTSpice, I use the 2N7000 because the BS170 is not available in LTSpice an it has very similar values. The simulation also aligns quite well with the results from the laboratory. In the lab, I used the correct MOSFET, namely the BS170.
IMG_4501.jpg
Screenshot 2023-11-25 at 12.54.00.jpeg.pngScreenshot 2023-11-25 at 12.53.24.png
 

Ian0

Joined Aug 7, 2020
9,503
Use a 74LVC1G17.
Maximum positive threshold is <2.2V for a 3.3V supply. It has enough output current to drive an LED.
 

crutschow

Joined Mar 14, 2008
34,047
why I am experiencing such distortion at high frequencies.
There are factors that affect switching speed other than the MOSFET speed.
You are not accounting for the stray capacitances in the circuit including the capacitances of the MOSFET.
1700923893395.png
You can reduce the value of the drain resistance to minimize its effect (below).

1700923614927.png
 
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