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12 votes

Is it misleading to say a common emitter amplifier is “out-of-phase”?

Whether you say that a signal is inverted, or phase shifted by 180°, depends on context. In the context of frequency response, where we are interested in the output of a system in respsonse to an ...
Simon Fitch's user avatar
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11 votes

BJT as current buffer for op amp

I'm sorry but pursuing that approach is probably not going to work for two reasons: - DC offsets will cause problems with an inductive load (can be solved) Not enough supply voltage to get 5 volts p-...
Andy aka's user avatar
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10 votes
Accepted

What causes signal distortion in the BJT circuit I designed?

Your biasing is too close to ground. Here is your circuit simulate this circuit – Schematic created using CircuitLab Here is a simulation when Vbias is 1 V vs when Vbias is 6 V. What happens ...
Math Keeps Me Busy's user avatar
8 votes

Why is the collector swing greater than the power-supply voltage?

You have inductance, and when you turn off the transistor, the inductor magnetic field must discharge and it will generate voltage of v=L*di/dt. You also have a 10mA constant current source, and it ...
Justme's user avatar
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8 votes

Behavior of BJT collector current in saturation

You are basically asking what will happen if you try to exceed the absolute maximum collector current. First, hFE is not constant. It will decrease substantially at high collector current, so there's ...
Spehro Pefhany's user avatar
8 votes
Accepted

Why is the frequency and voltage of my Colpitts oscillator lower on breadboard than LTSpice?

A breadboard will have capacitance between the connection rows. The jumper wires will have inductance. Real components will have parasitic resistance, inductance and capacitance that a simulation may ...
GodJihyo's user avatar
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7 votes
Accepted

How can I manually analyse this simple BJT circuit?

You know that \$V_b=+15\:\text{V}\$. You know you have approximately \$35\:\text{V}\$ across the \$10\:\text{k}\Omega\$ emitter resistor, so the emitter current will be slightly less than \$3.5\:\text{...
periblepsis's user avatar
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7 votes

BJT B-E Zenering: whither the collector?

There's an old riddle I think posed by Bob Pease on his magazine column, back in the day: how to obtain negative voltage from few ordinary components (BJT included) without oscillation? The quirk is, ...
Tim Williams's user avatar
7 votes

BJT as current buffer for op amp

Is there any method which can be used to achieve the 10-20Khz 0-5V sine wave Using 5V supply? A class D amplifier IC will do it, but it won't be exactly a 0-5V sine wave due to resistive drops in ...
Kuba hasn't forgotten Monica's user avatar
7 votes
Accepted

BJT four-resistor bias circuit analysis, parallel resistors

Thevenin's theorem applies as follows:
Franc's user avatar
  • 1,414
7 votes

How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

the relation Ic = β x Ib would lead me to believe that the collector current would remain the same, but this obviously cannot be the case. What am I not understanding? Consider the standard ...
Andy aka's user avatar
  • 460k
7 votes

What causes signal distortion in the BJT circuit I designed?

You are showing us an emitter follower (or common collector) topology. You've included a DC bias to your base source, which is good to do. But the problem is that while \$Q_1\$ can source current to ...
periblepsis's user avatar
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6 votes

IR LED with transistor

With 5V power to the LEDs, 0.7V across LEDs (as hypothesized in the question), 30Ω resistors, and a saturated BJT, we'd expect like (5-0.7-0.2)/30 ≈ 140mA per LED, thus ≈ 700mA total IC. With hFE = 40 ...
fgrieu's user avatar
  • 1,670
6 votes
Accepted

Cascaded BJT Amplifier

That resistor is to set the AC gain of the second stage. The AC gain of a common emitter amplifier is roughly the impedance in the collector circuit divided by the impedance in the emitter circuit. ...
GodJihyo's user avatar
  • 23.7k
6 votes

What prevents this transistor based discrete logic gate design from having been used?

Saturating logic is slow. Resistor based logic is slow, fairly power hungry, and not easy to integrate into ICs. Looking at the development history, the first type of 'standard' logic was RTL, ...
Neil_UK's user avatar
  • 167k
6 votes
Accepted

BJT Differential Amplifier problem

The motive Transistor differential amplifier (aka "transistor differential pair", "emitter-coupled pair", "long-tailed pair", etc.) is an ingenius circuit solution that ...
Circuit fantasist's user avatar
6 votes

Why would this circuit have a "maximum" output power?

The shunt regulator attempts to regulate the output to 9.4V. Now if the output voltage falls to less than 9.4V the shunt regulator is inactive. The maximum power the circuit can deliver to the load if ...
Spehro Pefhany's user avatar
6 votes

Multistage common-emitter 2N4401 amplifier

I mentioned using global NFB in comments, earlier. I also gather from your approach that you are comfortable with standard CE BJT stages and comfortable with the idea of two such stages. So let's take ...
periblepsis's user avatar
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6 votes
Accepted

Why am I getting heavy output distortion from a BJT amplifier when driving an 8 ohm load?

I'm trying to build a simple BJT amplifier for my electric guitar that would drive a 1 Watt 8 Ohm speaker Firstly, fix the output stage of your push-pull amplifier: - Next, the most significant ...
Andy aka's user avatar
  • 460k
6 votes

Behavior of BJT collector current in saturation

What 'saturation' means is that the external circuit is controlling the current, not the beta*Ib of the transistor. Any particular transistor might be able to conduct 10 A if it has low enough ...
Neil_UK's user avatar
  • 167k
6 votes

Equivalent resistance calculation to find lower cutoff frequency of the transistor

If you replace the transistor by its hybrid-\$\pi\$ model then you should see how the various resistances take place: I am applying here the fast analytical circuits techniques or FACTs as described ...
Verbal Kint's user avatar
  • 22.4k
6 votes

BJT B-E Zenering: whither the collector?

This is how I simulate reverse breakdown of base-emitter junctions: simulate this circuit – Schematic created using CircuitLab Diodes D2 and D4 block current when the B-E junction is forward ...
Simon Fitch's user avatar
  • 36.5k
6 votes

Saturation region of BJT

"Saturation" is confusing. It has at least three different meanings in electronics. "Saturation current" is the theoretical diode current under reverse bias in the diode equation. ...
John Doty's user avatar
  • 4,432
6 votes
Accepted

TAOE3, Exercise 2.13 -> Verify that a 8°C rise will cause grounded emitter BJT to saturate

Straight from Ebers-Moll: $$ \frac{I_{C_2}}{I_{C_1}}=\exp\left(\frac{\Delta V_{_\text{BE}}}{V_T}\right)$$ (See derivation below.) So, assuming about \$-2.1\:\text{mV}\$ change in \$V_{_\text{BE}}\$ ...
periblepsis's user avatar
  • 9,828
6 votes

BJT four-resistor bias circuit analysis, parallel resistors

Let me provide a different way of looking at it. It may be simpler to see your question with regard to \$R_1\$ and \$R_2\$ using math. Let's keep the BJT base current in place and assume that it is ...
periblepsis's user avatar
  • 9,828
5 votes

Does changing the collector resistance of a common base amplifier have any effect on the current?

If you concentrate on the VBB, RE part of the circuit, then the transistor behaves as a current source into the collector load. For an ideal current source, its output resistance is infinite, so the ...
Neil_UK's user avatar
  • 167k
5 votes

Is it misleading to say a common emitter amplifier is “out-of-phase”?

It's misleading when you're an absolute newbie, but as soon as you learn how to think in terms of frequency responses it makes as much or more sense than saying the signal is inverted. As with a lot ...
TimWescott's user avatar
  • 46.5k
5 votes
Accepted

IR LED with transistor

With \$h_{FE}=40\$, base current will have to be \$I_B=\frac{500mA}{40}=13mA\$ worst case, but there's no way you can expect the IO to source or sink that much current. By the way, your calculation ...
Simon Fitch's user avatar
  • 36.5k
5 votes

How is R3 drawing 0.6 mA current?

We can assume that in forward active region, the base-emitter junction of Q3 has 0.6 V across it (other authors might assume 0.7 V or some other value, but 0.6 V is a reasonable assumption). It would ...
The Photon's user avatar
  • 130k

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