low current transistor bias

[Kevin Weddle]

I have this schematic with unknown bipolars. The transistors are biased at an extremely low current and higher voltage. Without the ability to have a large change in collector current, as it's biased at like 1mA, the input signal and output signal are destined to be small. Even the final amps are this way. I think it's a trick. The output speaker that they feed must be very low power.

My question is this, what's wrong with mid current in a bipolar? I mean if the collector current changes by a small amount, then base change is extremely small. How reliable can that be? 

 

[audioguru2]

I have this schematic with unknown bipolars.

Please attach the circuit's schematic so we can see what you are talking about.

The transistors are biased at an extremely low current and higher voltage. Without the ability to have a large change in collector current, as it's biased at like 1mA, the input signal and output signal are destined to be small.

No. The AC gain of a common emitter transistor amplifier stage is determined by the ratio of its collector load impedance to its emitter resistance, not its bias current. A transistor's DC gain is actually a little less at higher currents.

Even the final amps are this way. I think it's a trick. The output speaker that they feed must be very low power.

The bias current of the output transistors in an audio power amp has nothing to do with the amount of power it can deliver to a speaker. The bias current is set so that the output transistors are never simultaneaously completely turned off to reduce crossover distortion.

My question is this, what's wrong with mid current in a bipolar? I mean if the collector current changes by a small amount, then base change is extremely small. How reliable can that be?

 

[Kevin Weddle]

The schematic is a very large text book offering of an AM transceiver. It has a mic input to an antenna for transmit, then the same antenna input for receive to a speaker. It's almost all bipolar and they are probably of average current.

Your right about the gain. But I went a step further. The one I'm looking at has a collector resistor of 300 ohms, biased at like 1mA. The collector signal is change in collector current times the collector resistor. Small change times most resistor values leads to small output signal. And the input must be less.

Bias current set at 1mA can only go between 0mA and 2mA and stay linear.

I'll start by saying that too much current swing is bad. Okay. The problem with too low a current swing is that bad spots on a transistor consume a larger portion of the signal. Kick the current out more and the bad spot consumes less of the signal.

 

[audioguru2]

The one I'm looking at has a collector resistor of 300 ohms, biased at like 1mA.
Bias current set at 1mA can only go between 0mA and 2mA and stay linear.

What is 1mA, the transistor's collector current or its base bias current?
Please sketch the transistor's circuit.

If it was a small power transistor as a common-emitter stage without an emitter resistor, with a current gain of 40, a 24V supply and its operating point at 12V for its collector, then its collector current is biased at 40mA and its base current is biased at 1mA. When its input current signal swings from 0mA to 2mA then its output voltage swings from 24V down to about 0.2V. No problem.

The problem with too low a current swing is that bad spots on a transistor consume a larger portion of the signal. Kick the current out more and the bad spot consumes less of the signal.

What are you talking about? Transistor current is fairly linear without any "bad spots". A transistor is non-linear when it is fed an AC voltage input instead of an AC current input. Negative feedback can take care of the non-linearity. 

 

[Kevin Weddle]

No, the collector current is like 1mA. How do you mean an AC current versus AC voltage?

Bad spots are manufacturing shortcomings. I really don't know much about the physics of transistors, but I can tell you all is not perfect. When I first started electronics I studied the book, then I used a digital voltmeter on the circuits I built and realized that this stuff was about hard work and constant tinkering. There's no way a shelf bought transistor can be all that.

 

[audioguru2]

No, the collector current is like 1mA.

With a 300 ohm collector resistor and only 1mA of collector current then the transistor is nearly cutoff and is not a linear amplifier. It mifgt be used to amplify a pulse that turns it on more.

How do you mean an AC current versus AC voltage?

A bipolar transistor has fairly low distortion when its base is driven with variable current. If it is driven with variable voltage (transconductance) then it is extremely unlinear.

Bad spots are manufacturing shortcomings. I really don't know much about the physics of transistors, but I can tell you all is not perfect. When I first started electronics I studied the book, then I used a digital voltmeter on the circuits I built and realized that this stuff was about hard work and constant tinkering. There's no way a shelf bought transistor can be all that.

Off the shelf transistors work fine when you operate them within the spec's in their datasheet. Add some negative feedback and transistors are just about perfect. Tinkering is not required when you understand the variations between transistors with the same part number and design the circuit to operate with those variations.

 

[Kevin Weddle]

Without a lot of thought, I find the variable current voltage thing hard to imagine. I understand what your saying. It reminds me of the voltage source versus current source thing. Thanks for the tip, I think...