# Problem with the fall time of a NPN transistor

I have the below circuit to translate a slow digital MCU on/off (3.3V) signal to a higher voltage signal (range from 12 to 24V). I have a specific attention for the Vin=12Vdc case, the voltage drop from Vin to Vsignalout shouldn't be more than 2V (extra safety guard is always good). With the below circuit I have about 1.7V drop that is ok.

But I have a big problem with the signal fall time is about 200 us that is too much (acceptable level should be about <80 us for my purpose).

This is the signal monitor for the above circuit (R2 is 1k):

The green signal represents the base signal of U$2 (now I see, I could rename it as T2 anyway), and the yellow one represents the signal at the collector. Below one is for R2 is 100 k: (The green probe is on the collector this time) First, I thought lowering the current helped the fall time, but e.g., I tried with 1 M ohm that had the same signal. I am not really sure why my fall time is terrible and how to fix the problem. Do I make a mistake by not using an extra base resistor for U$2?

Here are the datasheet of the transistors: U$2: PZT2222AT1G Q1: BCW66GLT1 One interesting thing is that I use MJD31C for the same circuit configuration that gets quite decent signal (but the voltage drop is too much that I am trying to replace it with PZT222A). I got stuck on it and would highly appreciate for any suggestion I was playing with R1 resistor by lowering the value to let it conduct more current, placed 47 ohm, but didn't help (MCU's GPIO has 2mA limitation). If anyone asks U$1 is a LDO to limit the output voltage to 15V, and the end diode is a CLD to limit the output current, I tested with them by removing and short circuit their paths, but it didn't help the signal ( my circuit should work with those components anyway).

• You'd be better off with a PNP e.g. MJD32C with series , Shunt R's req'd – Tony Stewart Sunnyskyguy EE75 Aug 24 '18 at 13:39