Different results with PNP and NPN, looking for explanation

Thread Starter


Joined Jul 8, 2010
I'm driving a piece of LCD glass, no backlight, which draws 0.1uA measured with multimeter.
circuit (NPN.jpg) is attached. The input is a 5V PWM signal. The output at the collector is ground, no switching. +V is +12V, but I've tried higher voltages.
When I use a PNP, same input, the output is the same PWM signal but at about 1V.
Why isn't the PWM signal duplicating at the collector?

The LCD glass must be the issue (I do not have a spec sheet), as using an LED gives a similar signal at collector as base.
Thanks in advance.



Joined Nov 30, 2010
The diode is allowing unlimited current to the transistor. You probably destroyed the transistor.


Joined Aug 1, 2013
With such a high load impedance, there is nothing to pull up the collector when the transistor is turned off. Replace the LCD with a resistor, 1K to 10K to start, and see if that gets you an output waveform. Then reconnect the LCD while leaving the resistor in the circuit and see if the waveform changes.



Joined May 4, 2013
Your drawing says "LCD Glass". Is this just the glass or is there a controller as well?

If it is just the glass, you need to drive it with ac, not DC. If you use DC, the LCD will fail fairly quickly from electroplating.

If it is an LCD shutter then you can drive it with a square wave and 2 inverters or the Q and Q-bar of a flip-flop. Drive the LCD from 2 outputs that are 180 degrees out of phase from each other. Note that the equivalent DC offset to the LCD must be small -- on the order of 50 mV. This requires a square wave that is right at 50% duty cycle such as the output of a toggle flip-flop. Inverters such as a 74HC04 or 74HC14 or a 74HC74 flip-flop can be used.