I couldn't post a thread on LTSPICE group about the subject, I'm too stupid to do so.
So I ask here, sorry.
I can't understand why "changing beta of a NPN BJT" does globally nothing about the collector current although it does in the real world.
So I ask here, sorry.
I can't understand why "changing beta of a NPN BJT" does globally nothing about the collector current although it does in the real world.
It all depends on how a transistor is wired into a circuit. If the circuit has resistors which establish an emitter current from a base voltage the current will only slightly change with transistor gain. The degree of change depends on how well the current is set. If only a base current is fed into a transistor then the collector current will vary wildly with the gain. That is no use as an amplifier circuit nor much else really. But it is the basis of a simple gain measurement. Put in a small base current and see how much collector current you get.
Good circuits are designed to work with a range of gains in the transistor where the emitter resistor or a current source sets the emitter current, whatever the gain of the transistor. So the base current may vary but the collector current is held more or less constant for different transistors by proper design.
But if you run a simulation using a particular transistor then the model will have a particular gain built in. You will not see an effect of gain change unless you actually use a different model. Not many transistors come with different gain models, but you can get some idea using, say, BC547A, BC547B and BC547C if those are available, as the models should represent the middle of each of the three groups of gains.
Good circuits are designed to work with a range of gains in the transistor where the emitter resistor or a current source sets the emitter current, whatever the gain of the transistor. So the base current may vary but the collector current is held more or less constant for different transistors by proper design.
But if you run a simulation using a particular transistor then the model will have a particular gain built in. You will not see an effect of gain change unless you actually use a different model. Not many transistors come with different gain models, but you can get some idea using, say, BC547A, BC547B and BC547C if those are available, as the models should represent the middle of each of the three groups of gains.
Thank you very much John.
Your explanations are perfectly clear.
I changed a MJ15030 with a BC547a in my design and here, the current was multiplied by 20.
I can't believe it.
In fact, as you understood, I was just changing the BF parameter of the original BJT.
I'm very pleased today.
At 77 I've learn something about amplifiers but there is yet a lot to work on.
Congratulations.
Your explanations are perfectly clear.
I changed a MJ15030 with a BC547a in my design and here, the current was multiplied by 20.
I can't believe it.
In fact, as you understood, I was just changing the BF parameter of the original BJT.
I'm very pleased today.
At 77 I've learn something about amplifiers but there is yet a lot to work on.
Congratulations.