Transistor saturation in an LED grid

[boylesg]

Suppose you bias a transistor with a single base resistor such that it will go into saturation with the maximum base current, as set by the base resistor and input voltage.

For example a 5V supply, a BC327 and a 1k base resistor - allowing a maximum base current of about 5mA

What happens if you then put say only 0.1mA through the base? Is it still regarded as saturated with a minimal CE voltage drop?

This could be the situation in a RGB LED grid where each individual led has a transistor controlling it (first tier) and then all of those transistors are further multiplexed with a second tier of transistors.

The second tier of transistor might have to 'turn on' one or more of the first tier transistors.

 

[Audioguru]

They cannot make transistors all the same. Some are better than others even when they have the same part number.

The datasheet for a transistor like the BC327 shows that ALL OF THEM saturate pretty well (Vce maximum of 0.7V) when the base current is 1/10th the collector current.
Some of them will saturate fairly well (Vce maximum of 1V) when the base current is 1/60th the collector current.

 

[boylesg]

They cannot make transistors all the same. Some are better than others even when they have the same part number.

The datasheet for a transistor like the BC327 shows that ALL OF THEM saturate pretty well (Vce maximum of 0.7V) when the base current is 1/10th the collector current.
Some of them will saturate fairly well (Vce maximum of 1V) when the base current is 1/60th the collector current.

I was not aware of saturation being a ratio of Ic to Ib.

I had been thinking that a transistor was saturated at a fixed Ib.

Well the data sheets make a little more sense then with various CE saturation voltages at corresponding Ib's.

Another gap in my knowledge filled so thanks.

So all I need to do is arrange Rb to allow saturation at just below the maximum Ic and then the transistors will sort themselves out (re individual Ib and Ic) at the various current draws of the LED cube.

 

[Audioguru]

Note that some manufacturers have more detailed datasheets than other manufacturers.
The BC327 datasheet from ON Semi has good detail and shows a graph of the "typical' saturation voltages.