# PNP transistor interface is not switching PWM inputs with low duty cycles

The following transistor circuit section(using a PNP transistor BC557B) is inverting and buffering a 25kHz PWM pulse train coming from one of the Nano's PWM supported digital pins. The following is the schematics diagram in LTspice:

As you see above, 25kHz PWM signal with 300ns ON time works fine in simulation.

In practice I tested this on a breadboard:

It looks a bit messy but it is exactly the same circuit. The 5V for the PNP transistor comes from Nano's +5V supply pin. I guess it supports up to 500mA or even some more.

So when I set the PWM to 25kHz and the ON time to around 300ns and when I probe the PWM output right before R4 the scope shows the following:

But at the output(probing between R3 and GND) the output is constant 5V as: (Sorry for the blur photo)

So the PNP obviously does not turn ON at this duty cycle unlike in simualtion.

Then I gradually increased the ON time of the PWM and it started to switch barely fine when the ON time was 1.5μs. Here is the scope screen for this at R4:

And below shows the output(at R3):

I tried to lower the values of both R4 and R3 down to 330 Ohm. But it didnt make any difference.

What could be the limiting the switching behavior of the transistor at low duty cycles here? Is the transistor not fast enough? What could be a remedy to lower this limit?

EDIT:

I found this PMOS and its SPICE model in a site as:

.model TP2104 PMOS (LEVEL=3 RS=0.73 NSUB=5.0E14 DELTA=0.1 KAPPA=0.0411 TPG=-1 CGDO=6.1716E-10 RD=1.91 VTO=-2.10 VMAX=3.0E6 ETA=0.12098 NFS=3.384E11 TOX=1.0E-7 LD=1.6980E-9 UO=865.45 XJ=6.5E-7 THETA=4.063E-5 CGSO=9.10E-9 L=2.5E-6 W=0.5E-2)


Then for a 10ns pulse(10 MegHz) input I compared the outputs in simulation for the BC557B and TP2104. Below shows the plots:

At least in simulation this particular PMOS worked better. But I didn't try it on breadboard yet.

• Show a graph with the input to the PNP and the output. – Voltage Spike Jul 17 '19 at 17:46
• What is the propagation delay between input and output? As in post a scope shot of input AND output as @laptop2d says. Odds are your transistor is not fast enough. You may need a MOSFET. – StainlessSteelRat Jul 17 '19 at 17:58
• Or you may be having Cout issues on the port with large current drives, but changing the resistance should have changed that. – Voltage Spike Jul 17 '19 at 17:59
• @StainlessSteelRat Why does it matter the thing is under 1500ns the transistor does not switch at all. I showed adl those in the question. I dont have access to it at the moment so I dont know the prop delay. – user1999 Jul 17 '19 at 18:18
• @StainlessSteelRat Please see my edit. I tried a particular small signal PMOS for high speed switching applications. In simulation it gave much better results comparing to the PNP BC557B. – user1999 Jul 17 '19 at 21:21