For the circuit shown, I'm trying to figure out if I need a base resistor or not.
What I'm looking to accomplish is a simple logic inverter to hold a power amp's active low shutdown at logic low until the tube warms up to a stable temperature.
I'm planning to leverage an existing inrush current limiter to drive the shutdown imput. (The idea for the inrush limiter is to extend the life of the tube filament by letting it come to temperature slowly, based on what I read in this article: https://audioxpress.com/article/constant-current-for-heater-tubes-extend-the-life-of-your-amplifier-s-vacuum-tubes)
The tube filament and inrush limiter form a voltage divider. When the circuit is at room temperature, the ratio should be about 20:200, making the voltage at point A logic high. When the circuit comes up to operating temperature, the datasheet for the inrush limiter says its effect will be negiligible (I'm assuming near zero reistance) so the vast majority of the voltage wil be dropped across the tube heater. This makes point A logic low.
This arrangement is exactly opposite of the active low requirement of the power amp's active low shutdown input.
Researching transistor inverters, I came across several that look like what I have in the schematic, with Rb, Q1, and Rc.
My question is this… Do I need the base resistor?
I'm thinking no, because I'm technically using voltage divider bias, with the tube filament and the inrush limiter forming the resistor network. But, the current flowing through these elements is about 150 mA. Normally, the voltage divider resistors in this type of biasing are calculated to pass a lot less current. Is there any possible harm to the transistor with these small resistance values and this large current?
If I think in terms of current amplification, and the transistor's beta of about 200, the ratio of Ib to Ic should be 1:200. Ic is limited to about 1mA by Rc. Does this arrangement dictate the base current can never be more than 1/200th of 1mA? It would seem so to satisfy the 1:200 current amplification. Or, is it a one way function where Ib controls Ic, but limiting Ic does nothing to limit Rb? In other words, Rb is still needed to limit the base current to a reasonable value? This does not seem to fit with the idea of voltage divider biasing, but I have never tried it with such a low resistance voltage divider.
I would prefer to not blow up a transistor to find the answer. Can anyone tell me if the Ib:Ic ratio holds true both ways, or do I need Rb to limit base current?
Best Answer
I suggest you use any Nch FET to act as a constant current source as long as RdsOn <=1 ohm or so and not use an NTC thermistor. Inrush current limiters (NTC’s) are not intend to be run continuously as they transition at high temperatures which reduces lifespan.
I can use a simple 5mm RED AlGaAs LED as a Zener of 1.9V @ ~5mA to bias the NPN current limit to 150 mA with only a 0.5 Ohm filament current sense of 75mV (standard current sense voltage) to drive the base with about 630mV.
The pullup from LED 2.9V to base Vb=630 mV was simulated with a dim light bulb as load that is 80 Ohms hot.
With slight variations in every component, the LED to base R should be tuned between 2.1k and 2.8k if you prefer full power or reduced a tad, to extend the tube life even more x2 for every 10’C drop at some tradeoff with thermionic gain.
I invite all pro designers to peer review this design including @jonk.
One can easily soft start this with an e-cap on the gate of the FET. The constant current power risetime is 2s/div.
This can reduce 12.6Vdc ac ripple slightly by choosing to dissipate some power in the FET.