# Could someone tell me the errors of that TTL driver (PNP Transistors)?

#### FICS20

Joined Dec 29, 2020
7
Hi everyone,
I'm trying to rebuild a TTL driver. Because I'm using PNP transistors for I'm not sure if my calculations are correct.

The idea is to feed the circuit with a square wave with 5V "high" ( 4.5V to 5V) and 0V "low" (0V to 0.5V) at least 4mA. When 5V is the voltage input then T1 should be in cut-off. That's mean that R2 should be no more than 1K Ohm; the transistor starts to work with 0.5V and the suggested base current will be 0.5mA. The negative power supply will be load in reverse the pin diode, that it should be connected.
Now, when the voltage input is 0 or 0,3 V, then T1 is active and R1 should be around 1.8k Ohm; T1 has a beta of 30 and the Ic should be 100mA. With 100mA should be load in forward the pin diode. T2 should be in cutoff. R3 will be around (7V-0.7V)/100mA =63 Ohms. R4 should be high enough to neglect the Ic current. Now R4 is calculated taking the values of 0.7V+30V/100mA = 307 Ohms. ??? Now I think probably I'm taking the wrong values. I should take the values for the pin diode? If and Ir instead the values of Ic?
Please tell me what I'm doing wrong?.

#### ericgibbs

Joined Jan 29, 2010
11,629
hi FIC.
Welcome to AAC.
Is there a reason why you have chosen PNP rather than NPN transistors.?
E

#### FICS20

Joined Dec 29, 2020
7
Thanks! No really. I just have PNP transistors. But It should work...

#### andrewmm

Joined Feb 25, 2011
907
what do you mean by a TTL driver.

the circuit looks like its a complicated high voltage driver,
what do you mean by re build

#### FICS20

Joined Dec 29, 2020
7
what do you mean by a TTL driver.

the circuit looks like its a complicated high voltage driver,
what do you mean by re build
I saw that circuit in a book of microwave semiconductor engineering by White. I just set other values for the power supply. I don't know really what is the correct name, but it should transform the logic inputs to voltages and currents. The name is driver circuit for Pin diodes I guess.

#### ericgibbs

Joined Jan 29, 2010
11,629
hi F,
This LTSpice sim shows the voltages.
What voltage output are you expecting.?
E
Update,

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#### FICS20

Joined Dec 29, 2020
7
hi F,
This LTSpice sim shows the voltages.
What voltage output are you expecting.?
E
Update,
When Vin is 5V Vo should be 0V and when Vin is 0V Vo should be at least 0.8V.

#### bertus

Joined Apr 5, 2008
21,045
Hello,

The name is driver circuit for Pin diodes I guess.
Pin diodes can be used to control RF signals.
With hard switching they can let a RF signal pass or block, depending on the voltage on the pin diode.

Bertus

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#### ronsimpson

Joined Oct 7, 2019
1,125
I know it really does not matter; It is easy to see what happens if the schematic is drawn with the highest voltage at the top of the page and the most negative voltage at the bottom. Input on the left and output on the right. Just my opinion.

Red trace is collector of Q1. Vout in blue is 0.8V and -28V

Last edited:

#### FICS20

Joined Dec 29, 2020
7
Hi romsimpson,
Thanks for the idea. But I used the same simulation and I don't reach 0.8V. It just reaches 0.67V. Are you sure it was 0.8V?

#### FICS20

Joined Dec 29, 2020
7
Thanks everyone! I fixed it

#### ronsimpson

Joined Oct 7, 2019
1,125
Thanks everyone! I fixed it
How did you fix it?

I see what you see. I just looked at the graph but did not measure with a meter.
Q1 C-E = 0.1V
I also see no voltage Q2 B-E because there is no current flow.

#### MrChips

Joined Oct 2, 2009
22,531
Thanks everyone! I fixed it
AAC is all about sharing knowledge and experience. When you fix something it would be considerate of you to show what you did to fix the problem.

#### FICS20

Joined Dec 29, 2020
7
AAC is all about sharing knowledge and experience. When you fix something it would be considerate of you to show what you did to fix the problem.
OF course! I had to leave the lab. The thing was that I forgot to add the voltage Uce to calculate R3. It means that I just need to have a value like 60 Ohm for example. When Q1 is active Q2 should be cutoff. I'm going to apply those values in the circuit.