Why is my 2N2222 transistor not biasing?

vlad777 said:

audioguru I think this is the second time you stated that there needs to be plenty Vce for hfe to be correct.
Can you explain how this works?

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I don't make transistors, I just read the spec's in the datasheet and follow them.
My first job was as a circuit designer for Philips where some of their experienced engineers taught me the details about their transistors.

For the 2N2222A datasheet posted the hFE (this is DC current gain. hfe is AC current gain.) is a minimum of 100 at a collector current of 150mA and a VCE of 10V. The minimum hFE is only 50 at the same collector current but when VCE is only 1V. The max saturation voltage loss at a collector current of 150mA and a base current of 15mA is 0.3V and the "typical" loss is 0.15V.

The transistor (and most other transistors) has a max saturation voltage loss shown when the base current is 1/10th the collector current and a graph is shown for the "typical" saturation voltage loss when the base current is 1/10th the collector current.

The change in beta with voltage can be related to the collector impedance (Hoe). The collector looks like a constant-current source but it is not perfect and has a finite impedance (an ideal current source has infinite impedance). For a given base current this causes the collector current to increase some with an increase in collector voltage. This causes the apparent beta to increase with collector voltage.

crutschow said:

The change in beta with voltage can be related to the collector impedance (Hoe). The collector looks like a constant-current source but it is not perfect and has a finite impedance (an ideal current source has infinite impedance). For a given base current this causes the collector current to increase some with an increase in collector voltage. This causes the apparent beta to increase with collector voltage.

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Now you are talking about the "Early" effect that is very small.

audioguru said:

Now you are talking about the "Early" effect that is very small.

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I know it's small but I believe that's what contributes to the difference in beta with collector voltage.

Or do you have a better explanation?

I have a Philips 1968 databook for Silicon Transistors. It has curves with many constant base currents and with the collector-emitter voltage varied. Most transistors show the collector current increasing when the collector-emitter voltage increases but other transistors show nearly no change.

crutschow said:

The required base current to keep the transistor in saturation does indeed depend upon collector current. Why do you think otherwise? If you are not providing enough base current the transistor will come out of saturation as the collector current increases. Thus the reason for calculating the required base current at the maximum collector current.

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audioguru said:

The datasheet for all transistors list the beta (hFE) when the transistor has plenty of collector to emitter voltage so it is not saturated.
The datasheet for your transistor lists the hFE when the collector to emitter voltage (VCE) is 1V and 10V. It shows that the hFE is half when the VCE is only 1V. When it is saturated then the VCE is typically only 0.05V at low collector currents then the base current must be much higher than the hFE would calculate.

The max Collector-Emitter Saturation Voltage is listed with the base current at 1/10th the collector current because the hFE does not apply to a saturated transistor.

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You both just stated that in saturation the collector current is lower. This does not make sense to me. I thought that having the transistor in saturation meant that the maximum amount of current goes through the transistor as it is complete turned "ON". Why then would the collector current decrease when the transistor is in saturation?

So then let's say I have this 2N2222A transistor and I want to calculate how much base current is needed to operate it in saturation with the wireless transmitter connected as show in the schematic I drew... how would I go about doing this? All I know about the wireless transmitter is that it draws 2.2mA when I connect it to a voltage source (don't remember what voltage I set but I believe it was 19V).

hugoender said:

You both just stated that in saturation the collector current is lower. This does not make sense to me. I thought that having the transistor in saturation meant that the maximum amount of current goes through the transistor as it is complete turned "ON". Why then would the collector current decrease when the transistor is in saturation?

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The collector current when a transistor is saturated depends on Ohm's Law: the supply voltage divided by the load resistance because a saturated transistor is a switch that is turned on.

So then let's say I have this 2N2222A transistor and I want to calculate how much base current is needed to operate it in saturation with the wireless transmitter connected as show in the schematic I drew... how would I go about doing this? All I know about the wireless transmitter is that it draws 2.2mA when I connect it to a voltage source (don't remember what voltage I set but I believe it was 19V).

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A saturated transistor needs its base current to be 1/10th its collector current. It is that simple.

I don't know why you have a resistor in series with your transmitter's power supply. The transmitter might not work properly with it there.

audioguru said:

The collector current when a transistor is saturated depends on Ohm's Law: the supply voltage divided by the load resistance because a saturated transistor is a switch that is turned on.

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I understand that but when a transistor is in active mode it limits the current right? So the maximum current would occur in saturation.... oh wait I just reread both yours and crutschow's statements and realize I was reading them incorrectly.

A saturated transistor needs its base current to be 1/10th its collector current. It is that simple.

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I don't know why you have a resistor in series with your transmitter's power supply. The transmitter might not work properly with it there.[/QUOTE]

Alright so is this 1/10th "rule" for all transistors or does it depend on the transistor? And if it depends then how can I tell by looking at the datasheet what the base current to collector current should be for saturation?

That series resistor was there because originally I had a 19V supply and I wanted to drop the voltage to 12V since I know the wireless transmitter uses a 12V battery. Now I am powering it with a 9V battery so they are not needed and I actually forgot to remove them in the schematic.

I came in late, and missed the later posts. Beta is not quite a constant in any transistor. Depending on where in the bias it is, gain changes. This is why the 1/10 B to collector current rule exists, as a transistor goes into saturation the gain actually decreases. Transistors have linear areas in their curves, but they aren't actually linear

When saturated, a transistor is a turned on switch. It does not limit the current.

On the datasheet for every transistor there is a spec called Collector-Emitter Saturation Voltage. The max voltage is listed when the base current is 1/10th the collector current for almost all little transistors like the 2N3904. High gain European transistors like the BC547 show the max saturation voltage when the base current is 1/20th the collector current.

Power transistors need more base current than 1/10th. The 2N3055 spec's a max saturation voltage of 3V (pretty bad) when the base current is 1/3rd the collector current when the collector current is 10A.