I am trying to build a Schmitt trigger from a pair of BJTs to convert a sine to a square wave.

The sine wave is from a Colpitts oscillator running at over 150 MHz. I want to measure the frequency using something like the SN74LV4040, which is why I want to square the sine wave somewhat.

This is the relevant part of the Schmitt trigger (with V1 representing the oscillator). Emitter current was set to 2 mA, high voltage to 3 V, low voltage to 2 V.


simulate this circuit – Schematic created using CircuitLab

But this is the output I get in LTspice.

Sine wave output from Schmitt trigger

Why am I getting a sine wave output from the Schmitt trigger?

This is the whole schematic (with some voltages plotted), if that helps:

Enter image description here

  • 2
    \$\begingroup\$ 150MHz isn't what 74 series capable of. \$\endgroup\$
    – Long Pham
    Commented Aug 26, 2018 at 11:45
  • 1
    \$\begingroup\$ Page 6 of the SN74LV4040 datasheet indicates a typical f(max) of over 200MHz. Is there another family that you'd recommend? \$\endgroup\$ Commented Aug 26, 2018 at 11:54
  • \$\begingroup\$ What happens if you run the simulation at a much lower frequency? Say, 50kHz? \$\endgroup\$
    – marcelm
    Commented Aug 26, 2018 at 11:54
  • \$\begingroup\$ Ah... Thankyou. It worked well at 1MHz \$\endgroup\$ Commented Aug 26, 2018 at 12:13
  • \$\begingroup\$ @talikarng You should check "switching characteristics" \$\endgroup\$
    – Long Pham
    Commented Aug 26, 2018 at 15:16

1 Answer 1


Kilo-ohms and over 100 MHz! You cannot get anything rectangular looking due the time constants caused by the parasitic capacitances, which can easily be 10 pF or more.

  • \$\begingroup\$ Thankyou. It worked when I ran the simulation at much lower frequencies. \$\endgroup\$ Commented Aug 26, 2018 at 12:13

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