transistor off

[elix]

anyone want to build a one transistor amp for headphones? i mostly to compare notes. rules would be you can do anything but you can have only 1 active device (either a mosfet or bjt), no op-amps allowed. i'd like everone to agree on one type of transistor (something generic) but thier probably not a good chance that everyone has the same types but if so that would be great. 


what do ya think?

 

[audioguru2]

No spec's?? Voltage gain. Distortion. Max output level.
It is very difficult to make a one-transistor amplifier for stereo.

The transistor's part number doesn't matter. A BC547 is the same as a 2N3904 (except the pins are reversed), and a BDxxx is the same as a TIP31.

If the supply voltage is fairly high and a trimpot can be used to fiddle with the bias for a Mosfet then the Mosfet might be best.

 

[elix]

anything go's, i have two irf530's waiting to be used but i don't want to go it alone 

 

[audioguru2]

Another guy in your class recently asked for help to make a headphones amplifier that uses a Mosfet. He actually tried to make it by himself before asking for help.

Why don't you give the circuit design a try then we can make siggestions to make it better.

 

[elix]

will post a schematic in a day.

 

[elix]

ok, i think i understand mosfets when they are in saturation mode but i'm having trouble figuring how to calculate gate voltage (Vgs) to drain current (Id) using G(transconductance ratio). in the schematic i just left generic values, i have a led in the place of r2 and i'm making a plot, but for now this is good enough, bed time 

 

[elix]

ok i have 3 points

gate voltage  current gain  Vd

4.6                  2ma              1.8
5.2                  13ma            2.2
5.6                  25ma            2.45

 

[elix]

gate V to I is almost 1:1, so what is G used for?

 

[elix]

using split supplys if i keep the gate at 4 volts above drain it saturates, when gate equals 0~4 volts above drain it stays in linear mode (corresponds to Vgs for irfz44, which is what i'm testing). Id=2k[{(Vgs-Vt)Vds-Vds^2}/2] is how you calculate drain current in linear mode but here's my questions:

Vt should be what?
is Vds the drop between drain and source? it shows up as one diode drop if so.
how do you figure out what k is?

 

[elix]

i'll answer one, Vt is the voltage threshold, should be 0 to start off with but if you reverse bias the gate current flows backwards through it 

 

[audioguru2]

A Mosfet turns on with a voltage between its gate and its source. When it is turned on then a current flows fbetween its drain and its source, the amout of current depends on the Mosfet (they are all different, even it they have the same part number), the gate to source voltage, the load resistance and the power supply voltage for the load.

Because they are all different, you cannot say that a certain gate voltage produces a certain drain current because only some of them will. You can manually adjust the gate voltage to match your Mosfet or you can use a circuit that automatically adjusts it.

The threshold voltage is the gate voltage that barely turns on the Mosfet. An IRFZ44 Mosfet has a drain current of 250uA with a gate voltage of 2V to 4V, each one is different.
The max current is 50A to 200A depending on the duration but the IRFZ44 has a minimum drain current of 31A if its gate voltage is 10V.

The gate of a Mosfet is a capacitor, it draws no DC current.

It is difficult to calculate the drain current because each one is different. The transconductance is shown only for "typical" ones on a graph and varies with drain current. 

 

[elix]

"IRFZ44 has a minimum drain current of 31A if its gate voltage is 10V"

so does it matter what the drain voltage is?

 

[audioguru2]

"IRFZ44 has a minimum drain current of 31A if its gate voltage is 10V"

so does it matter what the drain voltage is?

The datasheet shows the current stays at about 33A if the drain voltage changes from 3V to 25V.
It is a "typical" Mosfet so the gate voltage is only 5.5V instead of 10V.

 

[elix]

thanks audioguru, i'll now proccede to made an amp.

 

[elix]

URL=http://imageshack.us]

not bad considering i used whatever was laying around, tell me how i can improve it, spacificly how can i get the mosfet in the feedback loop?

 

[elix]

direct link,

 

[audioguru2]

Hi Elix,
Your opamp has a very low input impedance of only 1.5k ohms which is too low for many audio sources.
The Mosfet has a very high voltage gain at low frequencies but there is no volume control.
If negative feedback is added then I think the poor high frequency response of the LM317 regulator will cause oscillation.

I found a simple Mosfet headphones driver in Google. It doesn't have voltage gain.
http://headwize.com/projects/szeke1_prj.htm

 

[elix]

i'll be making some changes, keep the comments comming. 

 

[audioguru2]

I think a class-A power amplifier is a crazy way to heat your house. It gets hot even if it doesn't have a signal.
I was wrong before. The Mosfet won't have a high voltage gain because the low resistance of the load is its drain load, not the very high impedance of the LM317 current source.