transistor gain for circuits

[monkey2410]

I am planning to create some of my own distortion pedal circuits however I am currently stuck with how you calculate a circuit to suit each different type of transistor. I have worked out (or at least i think i have) how to calculate transistor gain in a circuit which I think has worked out as it should other than the emitter current (Ie) is not what I was expecting. Ic is 0.9mA and Ib is 0.08mA, therefore Ie should surely show as 0.98mA right?? but in this case the Ie is jumping up and down between 0.3mA up to 0.75mA. I was wondering if there is different principles for working out transistor gain for different NPN transistors, otherwise I'm puzzled. And also if anyone could give me anymore advice for the best method to calculate new circuits for switching different transistors out then that would be a huge help.

[amptramp]

Correct on the 0.98 mA because the current has nowhere else to go.  If your Ie is changing, the other currents are changing too.  Your method of measurement may be a bit suspect here.  The sum of all currents going into and out of a device has to be zero.  This applies to every electronic device except an antenna.

Transistor gain varies with current and temperature and there is a large variation between devices.  Most amplifiers use a lot of feedback to stabilize the characteristics or circuit topologies like differential amplifiers where the current can be set external to the amplifying transistors.

[Transmogrifox]

Correct on the 0.98 mA because the current has nowhere else to go....  Your method of measurement may be a bit suspect here.  The sum of all currents going into and out of a device has to be zero. 

+1

This is not theoretical or hypothetical, it's concrete such as if you connect two hoses in a "Y" connector and measure all 3 flow rates, flow(out) = flow(in1) + flow(in2).

If you measure something else you have only these possibilities:
A) a leak
B) faulty meters or measuring method
C) The rates are changing rapidly, which points back to (B)

If IE is fluctuating then your measurement system is negatively impacting the operation of the circuit in some way.
You need to give more detail about how you have connected things and how you are measuring currents in order to get any more help.

[PRR]

Welcome.

> Ic is 0.9mA and Ib is 0.08mA

That is an exceptionally "poor" transistor. hFE is only 0.9/0.08 or 11. This was typical for 1956, but since then hFE usually runs 50 to 500, not 11.

[antonis]

if anyone could give me anymore advice for the best method to calculate new circuits for switching different transistors out then that would be a huge help.

Despite the absolutely correct points of the above responders, you have to study some basic principles about BJT amplifiers..

i.g. for given IN/OUT impedance & Gain, which is the Collector quiescent current, which is the most apropriate bias method, which is Emitter bias voltage e.t.c.

For switching different transistors circuits, any method INDEPENDENT of individual transistor gain should be acceptable..

[monkey2410]

if anyone could give me anymore advice for the best method to calculate new circuits for switching different transistors out then that would be a huge help.

i.g. for given IN/OUT impedance & Gain, which is the Collector quiescent current, which is the most apropriate bias method, which is Emitter bias voltage e.t.c.

For switching different transistors circuits, any method INDEPENDENT of individual transistor gain should be acceptable..

I will definitely take that on board and check those out, however the simple circuit i have at the minute works perfectly fine it was designed by another user for the use of a 2n3904 npn transistor. My point was more about trying to work out why my multimeter wasn't reading 0.98mA at the emitter when it was reading 0.9mA at the collector and 0.08mA at the base. And also me wanting to know what the best way was to calculate the circuits required component values if I was to replace the 2n3904 with a different transistor.

[monkey2410]

Okay so i've figured out why i couldn't measure the emitter current, its simply because my circuit is a common emitter amplifier circuit where the emitter is grounded therefore the current at the ground is being effected by other components in the circuit such as the 10uf cap i'm using 

would still appreciate information on the best method to calculate new values for resistors/caps in my circuit based on the different transistor models i replace my 2n3904 with (this is for a distortion pedal so i need to achieve an output signal that wont be too low that i have to crank the gain up full to hear anything but also not too much that the signal clips easily with a strum. Is there a certain output current i should be trying to achieve for each pedal circuit?)

[PRR]

> 2n3904 ...reading 0.9mA at the collector and 0.08mA at the base.

I say again: something is wrong with your measurements. A 2N3904 with 0.9mA in collector would be expected to have like 0.005mA in Base. To measure a current you have to *break* the circuit to put your ammeter in. Since this is awkward, we almost always measure Voltage across a resistor and infer the current.

> what the best way was to calculate the circuits required component values if I was to replace the 2n3904

In a "good design", the resistors tell the transistor what to do. Within limits, you can change the transistor without any other change. Since the '3904 is readily available, cheap, and a good performer, I do not know why you would want to change it.

[antonis]

Okay so i've figured out why i couldn't measure the emitter current, its simply because my circuit is a common emitter amplifier circuit where the emitter is grounded therefore the current at the ground is being effected by other components in the circuit such as the 10uf cap i'm using 

Despite the placing of 10μF cap in your circuit, it shouldn't affect any DC measurement..
(even if you have a CE amplifier with Emitter degeneration and Emitter resistor by-pass cap, it shouldn't deal with bias quiescent (idle) current..)

[monkey2410]

Might be easier if i just show you my circuit at the minute then maybe you guys will see the problem?


The reason why im wanting to switch transistors out is because I am doing tests to see how different components will effect the the characteristic of the distortion produced, (I will at some point be making different methods of clipping too however at the moment I am struggling to calculate the circuit with other transistors so..)

Here is a sketch of the circuit:

[GibsonGM]

Are you intending to have the transistor itself be part of the distortion?  One stage probably isn't enough for that.

What is often (but not always) the case, you want to get max clean boost out of the component (transistor, opamp etc), and then hit the diodes with it...with variance for adjusting that gain (drive pot).   If it will not produce enough boost for you, you add a second stage.   It is often nice to KEEP the active device from clipping....you control it with the diodes.   

There are exceptions, such as transistor fuzz-type things, and JFET/MOSFET preamps where you feed a hot signal to the device ahead to intentionally clip it...what you're showing shouldn't cause the transistor to clip, tho...so it's just a bigger signal than what you input that is hitting the diodes....

A lower gain transistor will just make that signal lower in voltage....

[antonis]

From your sketch we have to presume that you have a grounded Emitter amplifier so amp's gain is calculated via R_C/r_e (Collector resistor/transistor intrinsic resistor) where r_e equals to 25 Ohm/I_C(mA) and R_C equals to Collector nominal resistor value in parallel with load's resistance..

In your case is about 180 which is in accordance with the rule of thumb for grounded Emitter amplifier gain: "20 times Vcc" (supposing that Collector is biased at Vcc/2 and ignoring out loading..)

With a rough estimation, any transistor with current gain (β or h_FE) greater than actuall circuit current gain (I_C/I_B) should be suitable for your circuit in the mean of as long as transistor current gain is greater than actuall circuit current gain you don't drive transistor to saturation..

[antonis]

you want to get max clean boost out of the component (transistor, opamp etc), and then hit the diodes with it...with variance for adjusting that gain (drive pot).   If it will not produce enough boost for you, you add a second stage.   It is often nice to KEEP the active device from clipping....you control it with the diodes.

Do I hear a typicall surreptitious Hi-Fi purist...?? 

[PRR]

It distorts first because the input goes right to a B-E junction. Signal levels much over 20mV (most guitar signals are) will be clipped. However this is also a low impedance point. Estimate 5K. The naked guitar is 5K bass-mids and 100K in highs. So it loads the highs much more than the bass-mid. Treble cut. This is OK, because the clipping adds a lot of highs.

At this point the collector swing is nearly 9V, except the two diodes will clip that to about 0.6V. Or working backward: 600mV out, gain near 160, means input levels above about 4mV (nearly all useful guitar signals) will be smashed.

> Ic is 0.9mA and Ib is 0.08mA

Now we see how you got these numbers. You did not read current *through* the transistor. You put the current meter *across* the transistor, gave current an easier place to flow, and measured the maximum current possible for this voltage and these resistances.



9V through 10K is indeed 0.9mA. 9V through 110K is 0.0818mA. You measured the resistors, not the transistor in action.

To measure current accurately you *break* the circuit and complete it with the current meter. This is awkward.

We usually just read the Voltages, and determine the current indirectly. For example, if Base is at 0.6V and Collector is at 1.6V, there is obviously 1.0V across the 100K resistor. The resistor must flow 0.01mA. This current has no place to go except the Base. So the base current must be 0.01mA. Similar thinking says the 10K has 7.4V, and is flowing 0.74mA. We already know 0.01mA flows to the 100K, so 0.73mA must go to collector. We usually just "trust" that both currents come together (where else could they go?) to give 0.74mA in emitter. If in doubt, we mod-in a small emitter resistor and measure there.

> how different components will effect the characteristic

I've seen this one before. The 10K drives the load and is a fine value for all guitar-cord work. The major "tweak" will be the ratio between this and the base resistor, because this sets the idle collector voltage and thus the symmetrical swing. A not-wrong setting is "about half". This nearly happens when Rb is hFE times bigger than Rc. High-hFE parts will want larger Rb.

"About half" bias is not necessarily the mangle sound you want. I see that for any likely transistor, your collector voltage is near 1.2V-1.3V, also a fine point. When you find a sweet spot, note the voltage. Then with other transistors, diddle Rb to get near the same voltage.

The next hack is those cap values. As-is they pass full bass and if over-driven will stay mis-biased long after a loud sound. That's fine. But there are other dynamics with smaller caps.

Finally, a 10K-100K series resistor at the input will reduce guitar loading and reduce distortion. With most guitars you get series resistance by turning-down the guitar. This is a handy way to "clean up". If you never want MAX! distortion, put some resistance in the pedal.

[GibsonGM]

you want to get max clean boost out of the component (transistor, opamp etc), and then hit the diodes with it...with variance for adjusting that gain (drive pot).   If it will not produce enough boost for you, you add a second stage.   It is often nice to KEEP the active device from clipping....you control it with the diodes.

Do I hear a typicall surreptitious Hi-Fi purist...?? 

No, Tony, no purist here (ha ha)...I thought he wanted to test out how devices DRIVING his diodes would differ (?)   I'm still not sure what he's trying to get at, but experimenting is awesome.  I thought he wanted to hit the diodes with a boosted but not clipped signal...

I'm all for a transistor driving a transistor driving a transistor!!     But to me, that's "fuzz", it goes very square wave (won't go too far into this idea lest I open a can of "device clipping comparison" worms) ...which is ok, but a one-trick pony.  My opinion.

The magic (to me) lies in JFETs or MOSFETs cascaded, quite similar to how you set up a nice chain of gain in a tube preamp!  No diodes required! 


Thanks for that write-up, Paul!  Awesome.   I did not see that a base resistor was required to stop the B-E junction from distorting...nice...