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I was going to make a voltage amplifier for weak signals with 2SD786, but I noticed that the manufacturers haven't included the characteristic curves in the datasheets. How can I optimize the circuit if I don't know this transistor well? For example, according to the datasheet \$h_{FE}\$ value is very different for different conditions. Should I try to generate the curves myself before using this transistor?

UPDATE:

Assume that for a given \$V_{CC}\$, you want to choose a good load line by choosing \$R_L\$. Also you should choose a good operating point on the load line by determining appropriate \$I_B\$. However you don't have the characteristic curves.

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    \$\begingroup\$ What you need to identify is the circuit of the amplifier you might wish to use with this transistor. The actual target circuit defines how the transistor is used and whether the hFE limitations (as seen in all BJTs) might unduly affect desired performance. In other words, with a proper design, the transistor limitations are largely eradicated; with a poor design you have side-effects due to transistor limitations. \$\endgroup\$ – Andy aka Sep 2 '18 at 13:57
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    \$\begingroup\$ Nice find - this would be a good preamp for a low-impedance signal source (likely not-so-good for a high-Z source). Data sheet suggests that low voltage noise requires a DC bias @ 10mA collector current. You can expect a fairly low amplifier input impedance biased this way. Also - be aware that many bipolar transistors meant for higher-power applications don't spec noise at all - and are at least as good as this one. \$\endgroup\$ – glen_geek Sep 2 '18 at 14:11
  • \$\begingroup\$ @Andyaka, I wanted to get the most possible gain from this transistor and at the same time avoid distortions. If I know the transistor well, I can choose the right values for the resistors. \$\endgroup\$ – Arham Sep 2 '18 at 14:18
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    \$\begingroup\$ Mutually exclusive requirements: most possible voltage gain and low distortion. More mutually exclusive requirements: most possible voltage gain and high temperature stability and drift. Show your circuit and take note of what will likely be said. \$\endgroup\$ – Andy aka Sep 2 '18 at 14:27
  • \$\begingroup\$ @Andyaka, please see the UPDATE paragraph. \$\endgroup\$ – Arham Sep 2 '18 at 15:12
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That's the reality of semiconductor manufacturing. Some parameters are really difficult to control tightly. Therefore, you design your circuits so that the overall circuit characteristics do not strongly depend on the actual characteristics of any particular device. Then, when you are done, you go back and verify that the circuit operation is acceptable over the full range of possible characteristics for each device.

It sounds complicated, but you'll soon learn how to subdivide the problem to keep things under control. You apply the same concept to multi-stage circuits — you design each stage so that its performance does not depend strongly on what is connected to its inputs and outputs.

For example, in an amplifier, you use negative feedback to set the gain of the circuit so that it is not a strong function of the actual current transfer ratio of the transistor. This could be as simple as putting a resistor in series with the emitter.

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These transistors are 'binned' so you know the hFE over a fairly narrow range. The 'R' parts have an hFE between 180 and 390 at 10mA Vce and 6V Vce, for example. So a good starting point would be 10mA Ic and design a bias arrangement to maintain the desired Vce.

I have no idea who "New Jersey Semiconductor" is, but those parts were originally made by Rohm of Japan (now discontinued). I still have a small cache of them. Maybe they specialize in parts that are no longer available and people will pay big bucks for.

Normally you would use negative feedback (for bias and for AC) and/or place the transistors in the negative feedback loop of a good low noise op-amp such as the '5532, which eliminates most of the effect of hFE variations. Similar considerations come into play with Idss of JFETs used in very low noise applications.

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