# Back to basics - BJT transistor BC547B (Solved)

#### PsySc0rpi0n

Joined Mar 4, 2014
1,545
Ok, in that case, no calcs should be needed. And I think this thread can be marked as solved. In the meantime, I'm going to try to recall and do the same for MOSFETs. I need a voltage amplifier... And when I started this with BJTs I didn't remember these were current amplifiers instead!

#### Audioguru again

Joined Oct 21, 2019
3,202
A transistor or a Mosfet can be a on-off switch or be a linear amplifier. They both can switch or amplify current, voltage or both.
If a transistor is switching or amplifying current, then Ohms law can show it also switching and amplifying voltage.
If a Mosfet is switching or amplifying voltage, then Ohm's Law can show it also switching and amplifying current.

#### sparky 1

Joined Nov 3, 2018
544
The algebraic relationship between V and I (Ohms Law) also makes it possible.
The logical conversion between current and voltage does use concept and calculation.

You can measure the voltage across the poles of the source or insert an ammeter in series with either pole.
Likewise you can insert a resistor in series to effect current or you can place a resistor in parallel with the source.
Series for current // parallel for voltage
When you measure the voltage across a resistor parallel to battery poles you have in effect converted to voltage.
When you measure the current after a resistor in series with battery poles you have in effect converted to current.
Both current and voltage are involved according to Ohms law. In order to make progress you need to work within
practicle range of values, developing an approximation for practicle BJT range, not a high impedace input of a jfet.
( my BJT circuit has enough current to demonstrate something practicle and is not outside quanitative limits for an application. )

Forget about the illusive theoretic voltage only event under extreme cold conditions does not prevent anyone from
learning the work done on electronic bench. The definition reads: V=I*R

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#### LvW

Joined Jun 13, 2013
1,247
@LvW I think you better start reading the posts until the end. I didn't start with Ib ever. I started with Ic and then used beta. But later, I fixed that approach and did the 10% thumb up rule.
Have a nice time. Bye.

#### PsySc0rpi0n

Joined Mar 4, 2014
1,545
My goal initially was to build a simple voltage amplifier and when I started searching I found the need to get back to basics of transistors. That's why I started this thread. But then, I (thought I) realised that MOSFETs would be better for amplifying voltage. Now, I'm not sure anymore.

@Audioguru again and @sparky 1 I know ohm's law and in terms of math, of course you can say that, but in terms of electronics, isn't one better suited for current amplifying and the other for voltage amplifying?

@LvW it's clear that you're not happy with what I said, but the truth is that you misquoted me a couple of times when it was not needed because I fixed/changed my approaches on later posts. So, you could have saved a few posts of yours by just replying to latest posts and just let go the older ones that were already out of context. That's all sir. Anyway, I really appreciate the help everyone gave.

#### Audioguru again

Joined Oct 21, 2019
3,202
A bipolar transistor can produce a very high voltage gain because a tiny base voltage signal can cause a high output voltage signal into a fairly high collector load resistance (but at high distortion).
A power Mosfet can produce almost infinite current gain because its gate current is zero but its output current can be very high.

#### LvW

Joined Jun 13, 2013
1,247
A bipolar transistor can produce a very high voltage gain because a tiny base voltage signal can cause a high output voltage signal into a fairly high collector load resistance (but at high distortion).
A power Mosfet can produce almost infinite current gain because its gate current is zero but its output current can be very high.
Comparing the two transistor types, I think the most logical way is to compare the transconductances of both devices. The "current gain" (a somewhat confusing term - in particular, for FETs) has an influence on the input resistance only, not on voltage gain.

#### LvW

Joined Jun 13, 2013
1,247
@LvW it's clear that you're not happy with what I said, but the truth is that you misquoted me a couple of times when it was not needed because I fixed/changed my approaches on later posts. So, you could have saved a few posts of yours by just replying to latest posts and just let go the older ones that were already out of context. That's all sir. Anyway, I really appreciate the help everyone gave.
Perhaps it would be helpful if you were a little clearer. So many misunderstandings could have been avoided.
Example (post'23):
"I didn't say that Ib sets IE. Or at least, I didn't mean that.
What I meant was that R1 and R2 were using the same voltage supply as the Collector terminal to activate the transistor. And therefore, Ib would be set by those 2 resistors. "
It is the task of the (low-resistive) voltage divider to produce a bias voltage at the base - and the corresponding base current will not be "set by those 2 resistors" but primarily determined by the transistor properties (base-emitter path).