modified 10 years ago

BJT Nor gate

01:02:30

Nor gate composed of resistors, diodes and BJTs  I found in my parts box (forward beta = 300 as is the average case for 2n2907 I checked for comparison).
- Minimum Vcc = 1.5V
- Maximum Vcc = 20kV
- Minimun output impedance (at Vcc >= 2V) = 20 Ohm
- Minimum output impedance (at Vcc < 2V) = 100Ohm
- Logic high output voltage >= 90% Vcc
- Logic low output voltage <= 10% Vcc
- Quiscent curren at logic low = Vcc/100000

SPDTs represent inputs, 700mOhm resistor represents internal power source resistance.

My design is crazy-inefficiant at logic high. A whole lot of power is disappated by the second BJT. If you have an idea how to resolve this I will be very happy (maybe som kind of feedback of the output voltage)

published 10 years ago

rbrtkurtz

10 years ago

2N2222 is an NPN transistor. 2N2907 is it's PNP equivalent.

spammailbox

10 years ago

Thanks, I got confused. Corrected

rbrtkurtz

10 years ago

As for the power, there isn't much power dissipation going on. It's only 49mA at 60mV collector-emitter and 10mA at 840mV base-emitter . That's less than 5mW on the collector side, and less than 10mW on the base side. Your load is doing all the dissipation. And that's where all the current is coming from. I assume that power dissipation you're talking about is the 59mA. Again, 10mA of that is going through the base, 49mA is through the collector to the load. You have 4.9V at the output, and a 100ohm load, so the current is 49mA. Ohms law. If you want less current draw, go with a higher resistance load. You could also increase the 100k resistor to limit current, but with both transistors having a gain of 300, the total gain is 90,000. It's going to take a lot of resistance to limit that. However, 300 gain for a 2222/2907 is pretty optimistic. At 125°C, 1mA Ice, 5V Vce, you *might* see a gain of 300. The gain goes up as temperature increases, and down as current increases. So you're probably looking at 200-250 on the gain at best.

spammailbox

10 years ago

@rbtkurtz, if the load impedence is very high (>100kOhmn), power is wasted like crazy (it gets even worse at higher source voltage)

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