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# Tag Info

## New answers tagged bjt

4 votes

### How to derive voltage gain in Common Emitter (CE) NPN BJT configuration?

derivation Assuming their simplified model for the collector current and a grounded emitter case, I'll get directly to your question and derive $A_v$ from the large-scale equations: \begin{align*} ...
• 12k
0 votes

### Push-pull BJT configuration can't pull output to ground fully

You can't get below 0.7 with devices that way. Exchange the NPN and PNP; put 2 10k in series between the bases and drive the midpoint. That'll get to about 0.1 V. For lower use mosfets - one CD4007 ...
• 19.8k
0 votes

### Push-pull BJT configuration can't pull output to ground fully

Are you sure you want pull to Gnd instead of high impedance? If the Gnd is true you can use a common emitter. When you use a Rc=1k and 100k to mixer the error should be less than 1%. At plus 3V the Rc ...
• 2,340
1 vote

### Diode-Connected load

MOSFET is a unidirectional device that varies its output drain current as a function of the input gate-source voltage, thus maintaining a relatively constant current. Jokingly, this is a “transistor-...
• 18.2k
2 votes

### Diode-Connected load

simulate this circuit – Schematic created using CircuitLab A MOSFET whose gate is connected to it's drain is called diode-connected. Edit: The VI characteristics of a diode connected MOSFET is ...
• 4,247
1 vote

### Advice for an AM detector using a BJT

Here is another trick for biasing a BJT near the cutoff current The current mirror here has been made out of Darlington pairs for increasing the input impedance. I have used the 2M2 collector ...
• 11
0 votes

### NPN transistor base doesn't drain as much current as I calculated

Why does collector resistance $R_C$ affect base current? The answer is it doesn't, at least not much. You've just got something wrong somewhere. There's always a slight dependence of anything on ...
• 42.4k
0 votes

### Why use diode connected BJT (and not just connect two BJT back-to-back) for current mirror circuit?

Current mirrors built with BJTs and MOSFETs have something to do with Integrated circuit design.In early integrated circuits, BJTs were used. Later on, MOSFETs were used. The use of a current mirror ...
• 908
5 votes

### Why use diode connected BJT (and not just connect two BJT back-to-back) for current mirror circuit?

The whole point of using another BJT with identical characteristics, is that they both agree on what $V_{BE}$ needs to be present in order to pass exactly 1mA, or any current. Your assumption that ...
• 42.4k
4 votes

### Base-emitter reverse breakdown and current

So my question is: considering that I'm breaking the absolute maximums here, what is the danger of the transistor actually breaking/degrading significantly? I just ran some tests with 2N3904 ...
• 26.2k
1 vote

### Why does current mirror need a diode connected configuration?

Short answer Connecting a transistor's gate to its drain introduces negative feedback, reversing its operation. Instead of converting voltage to current, it now converts current to voltage. By ...
• 18.2k
1 vote

### Why use diode connected BJT (and not just connect two BJT back-to-back) for current mirror circuit?

Short answer Connecting a transistor's base to its collector introduces negative feedback, reversing its operation. Instead of converting voltage to current, it now converts current to voltage. By ...
• 18.2k
0 votes

### NPN transistor base doesn't drain as much current as I calculated

Ignoring odd arrangements, there are two cases that have very different results with BJTs: (1) operated as an amplifier (active mode); and, (2) operated as a switch (saturated mode). Technically, ...
• 12k
0 votes

### Best Technique to Find 'The' BJT For Your Circuit?

Made with microcap v12 Note that UJC1210K is discontinued, it can be replaced with some equivalent. With the use of SiC MOSFET, you can try something like this circuit. The dissipated power is "...
• 15k
6 votes

### Solenoid driver circuit with peak hold

advantage of the BJT could be to choose a base current so the BJT also could be used as a current limiter It doesn't matter much if it would be a BJT or a MOSFET, since the control of the current has ...
0 votes

### NPN transistor base doesn't drain as much current as I calculated

Choosing a base current depends what the load is and what frequency the load is driven, another words how much time is allowable the Bjt stays in linear region during switching. A higher base current -...
• 2,340
0 votes

### NPN transistor base doesn't drain as much current as I calculated

The Vbe of the transistor will be about 0.7V so you can expect the base current to be roughly (5V-0.7V)/470Ω = 9.1mA. Since the base current is relatively high Vbe might be higher, and therefore the ...
6 votes

### Solenoid driver circuit with peak hold

Below is the LTspice sim of a PWM Hit & Hold circuit for the solenoid: It uses a CD4093 quad Schmitt-trigger NAND gate to generate a 10kHz PWM signal, which holds the solenoid at 10mA after the ...
0 votes

### Best Technique to Find 'The' BJT For Your Circuit?

You can try search & sorting tools on manufaturers sites also like On-semi. In your case I would filter according Ft parameter. Since the amp works in linear area the Ft/2 frequency is ussually ...
• 2,340
1 vote

### Positive and Negative Voltage Buffers with Current Buffers

It's not generally bad to reverse bias the base-emitter junction, and doing so doesn't automatically damage the transistor, unless that causes excessive power dissipation. There is apparently some \\$\...
• 42.4k
8 votes

### Base and emitter pins of BJT swapped

If I understood your problem properly, you have in your PCB pins 2 and 1 swapped, like this, but the BJT is as the diagram in your question shows: In that case, I would try to un-solder the BJT and ...
• 127
5 votes
Accepted

### Power latching circuit MOSFET is stuck ON

Power latching circuit MOSFET is stuck ON It's not stuck on. You've wired the P channel MOSFET incorrectly. The body diode will always conduct giving you the impression that it is "stuck on&...
• 467k
2 votes

### Current measurement with c BJT current mirror

Just change some resistor's values ... Note that the change of resistor of 50 Ohm is very "critical". Simulated at two temperatures with microcap v12. Gain is calculated with (my) R2/R1 (To ...
• 15k
2 votes
Accepted

### Current measurement with c BJT current mirror

By using 0.1 ohm shunt at 2A the drop across shunt is 0.2V. Setting gain of T1 with Rc/Re ratio to 21 you are getting 4.2V Vcurr (at 2A). You cannot set the circuit to output full 5V Vcurr since there ...
• 2,340
0 votes

### How does this amplifier stage work?

Q1 is functioning as an emitter follower, but most of the current is supplied by Q2. R3 along with R5 and RV1 form a voltage divider that sets the overall gain. You can think of it as a very simple ...
• 177k
2 votes

### RC Phase Oscillator Resistor placement

The second circuit doesn’t oscillate because there’s no feedback, you’ve connected C1 directly to your supply voltage.
• 26.1k

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