Transistor

Neewbie_noob

Need help with transistor experiment.

I have obtained data from an experiment and I have to show the relationship between voltage gain and input frequency.
I am expecting a graph like this



The on one the bottom left.

How ever, I am actually getting a graph which is very straight line and only one raised point, my lecturer me that we should use a log scale because of the scale of the numbers we are using. Frequency ranges from 500Hz to 10MHz, voltage gain starts at 2,peaks at 7 and falls to 0.08 V.

What do I plot against what to get the bell graph (or something that resembles it).

Cheers.

Michael Collins

Graphs (b) and (c) don't really make any sense - you'd hardly ever put frequency on the vertical axis like that, and even the plots themselves don't make sense. How does changing the frequency change the gate-source voltage Vgs (or how does changing Vgs change the frequency if you read it that way).

Maybe tell us what data you have available?

Graph (a) looks as you would expect.

krd

Michael Collins wrote: »

Graphs (b) and (c) don't really make any sense

No, it might make sense if we could see the circuit diagram.


The first graph shows a linear relationship between the current and frequency. The next two, show a curve within the voltage ranges. Which is not surprising for semiconductor material. And it's really horrible, if you have an exam, where the questions are like "and what kind of curve do you think you'll get from this circuit arrangement?".

So, from the graphs, it looks as though for the circuit, the frequency is driven by the input current, and the wobbles in the voltage show the difference in power needed at different frequencies.

Hysteresis.......hysteresis.

Neewbie_noob

krd wrote: »

No, it might make sense if we could see the circuit diagram.


The first graph shows a linear relationship between the current and frequency. The next two, show a curve within the voltage ranges. Which is not surprising for semiconductor material. And it's really horrible, if you have an exam, where the questions are like "and what kind of curve do you think you'll get from this circuit arrangement?".

So, from the graphs, it looks as though for the circuit, the frequency is driven by the input current, and the wobbles in the voltage show the difference in power needed at different frequencies.

Hysteresis.......hysteresis.

Can I attach my own image onto this thing to show you? How do I do it?

krd

Neewbie_noob wrote: »

Can I attach my own image onto this thing to show you? How do I do it?

Yep, go ahead.

It's worth a look.

Michael Collins

Looking at it again, that frequency axis is probably the transistion frequency [latex] f_{t} [/latex] i.e. where the current gain drops to unity (0 dB). So to get graph (b) you need to plot [latex] f_{t} [/latex] (on vertical) versus gate-source voltage [latex] v_{gs} [/latex] (on horizontal). You may have to work out [latex] f_{t} [/latex] first, you do this using graph (a).

Your data should include a list of current gains versus gate-source voltage (or other data from which this can be found).

If you can post what data you have we can take ya through it.

Neewbie_noob

Thanks sooo much guys and fellow boards nerds, turns out there was a decimal missing in one of my numbers, so instead of 5.96 Volts, it was 596 Volts, I have about 100 readings, buuttt it's all sorted now and I got the appropriate graph.

My voltage gains were of the order of 0-10 Volts, whereas my input frequencies were of the order of 500 - MegaHz .. but I have the graph now, which is sort of bell-shaped.

Thanks for everyone's help.

krd

Neewbie_noob wrote: »

Thanks sooo much guys and fellow boards nerds, turns out there was a decimal missing in one of my numbers, so instead of 5.96 Volts, it was 596 Volts, I have about 100 readings, buuttt it's all sorted now and I got the appropriate graph.

That's kind of a floob I would make. I'm not sure how you were taking your voltage. One thing, you might have noticed if you were really getting 596 Volts, as most low power transistors can't take much voltage - 20 or 50 V and they'll blow - I learned this by burning a few transistors.

Also, if you see a voltage much higher than your input voltage, and no part of your circuit is acting as a transformer, then your voltage reading is wrong.

Neewbie_noob

Frequency (Hz) V_out (Volts) V_gain (Volts) LOG[10] (Frequency)
500 2.4 60 2.698970004
600 2.88 72 2.77815125
700 3.2 80 2.84509804
800 3.6 90 2.903089987
900 3.84 96 2.954242509
1000 4.16 104 3
1500 5.04 126 3.176091259
2000 5.76 144 3.301029996
2500 6.16 154 3.397940009
3000 6.24 156 3.477121255
3500 6.4 160 3.544068044
4000 6.56 164 3.602059991
4500 6.64 166 3.653212514
5000 6.72 168 3.698970004
6000 6.8 170 3.77815125
7000 6.88 172 3.84509804
8000 6.96 174 3.903089987
9000 6.96 174 3.954242509
10000 6.96 174 4
20000 7.04 176 4.301029996
30000 7.04 176 4.477121255
40000 7.04 176 4.602059991
50000 7.04 176 4.698970004
100000 6.88 172 5
150000 7.12 178 5.176091259
200000 6.8 170 5.301029996
250000 6.64 166 5.397940009
300000 6.56 164 5.477121255
350000 6.4 160 5.544068044
400000 6.32 158 5.602059991
420000 6 150 5.62324929
440000 5.92 148 5.643452676
460000 5.92 148 5.662757832
480000 5.76 144 5.681241237
500000 5.68 142 5.698970004
520000 5.6 140 5.716003344
540000 5.52 138 5.73239376
560000 5.44 136 5.748188027
580000 5.36 134 5.763427994
600000 5.28 132 5.77815125
620000 5.2 130 5.792391689
640000 5.12 128 5.806179974
660000 5.04 126 5.819543936
680000 5.04 126 5.832508913
700000 4.96 124 5.84509804
720000 4.88 122 5.857332496
740000 4.8 120 5.86923172
760000 4.72 118 5.880813592
780000 4.72 118 5.892094603
800000 4.64 116 5.903089987
820000 4.56 114 5.913813852
840000 4.48 112 5.924279286
860000 4.48 112 5.934498451
880000 4.4 110 5.944482672
900000 4.32 108 5.954242509
920000 4.24 106 5.963787827
940000 4.16 104 5.973127854
960000 4.16 104 5.982271233
980000 4.08 102 5.991226076
1000000 4 100 6
1500000 3.04 76 6.176091259
2000000 2.32 58 6.301029996
2500000 1.92 48 6.397940009
3000000 1.6 40 6.477121255
3500000 1.44 36 6.544068044
4000000 1.2 30 6.602059991
4500000 1.12 28 6.653212514
5000000 0.88 22 6.698970004
5500000 0.88 22 6.740362689
6000000 0.8 20 6.77815125
6500000 0.64 16 6.812913357
7000000 0.56 14 6.84509804
7500000 0.56 14 6.875061263
8000000 0.48 12 6.903089987
9000000 0.48 12 6.954242509
10000000 0.048 1.2 7

results above



https://mail-attachment.googleusercontent.com/attachment/u/0/?ui=2&ik=8ddfcf67df&view=att&th=13a702be5d557ed2&attid=0.1&disp=inline&realattid=f_h8etsrbc0&safe=1&zw&saduie=AG9B_P8VTFxsX6Za9yWrnzk3R7HT&sadet=1351006579060&sads=39aBa5Ij49OEmiEUBuUgOLehoC0&sadssc=1

circuit diagram 2.4

WHY DOES THE VOLTAGE GO UP THEN DOWN EVEN THOUGH THE FREQUENCY GOES UP ??

Michael Collins

I can't access that circuit diagram, any chance you can post it up somewhere?

krd

Neewbie_noob wrote: »

WHY DOES THE VOLTAGE GO UP THEN DOWN EVEN THOUGH THE FREQUENCY GOES UP ??

That's a good question....Which I cannot remember the precise answer to. It's a hystersis effect. I think (and my advice is handwaving because I can't remember), as the current increases, the depletion layers in the transistor get fatter, this creates more resistance and draws more power. But dependant (on other factors I can't remember), as the frequency increases the depletion layer has less time to expand, and this lowers the resistance, so the power draw falls again.

And that's my Jazz hands version of what's happening.


And, we don't all have DIT logins.