Therefore there is no interaction between different inputs, and any number of diodes may be used. A disadvantage of this circuit is the input resistor to the transistor. Its presence tends to slow the circuit down, thus limiting the speed at which the transistor is able to switch states. At first glance, the NAND version shown on the left should eliminate this problem. Well, not quite.
Diodes exhibit a very similar forward voltage when they're conducting current.
Therefore, even with all inputs at ground, the transistor's base will be at about 0.65 volt, and the transistor can conduct. To solve this problem, we can add a diode in series with the transistor's base lead, as shown to the right. Now the forward voltage needed to turn the transistor on is 1.3 volts. For even more insurance, we could add a second series diode and require 1.95 volts to turn the transistor on. That way we can also be sure that temperature changes won't significantly affect the operation of the circuit.