# Tag Info

### Why do people use MOSFETs instead of BJTs in PWM applications?

Why do people use MOSFETs for PWM and not BJTs? Mostly because driving BJTs is a relative pain in spite of their lower input capacitance. They are current controlled devices, not voltage controlled, ...
1 vote

### Why do people use MOSFETs instead of BJTs in PWM applications?

The BJT is harder to drive than the MOSFET or IGBT. High current high voltage devices can have a current gain of less than 10. More complex circuitry like prop drive or baker clamps to keep the BJT ...
• 13.5k

### Why do people use MOSFETs instead of BJTs in PWM applications?

Be careful you do not mistake analog bandwidth vs digital switching speed. When you say or hear "BJTs are better at high frequencies", what is often being referred to is bandwidth as an ...
• 53.4k

### Can a small signal BJT handle small currents of reverse voltage (Vce) for a short time?

Q2 is safe because prospective currents are very low. C8 is not safe so you could put a reverse 1N4148 diode across it or use a MLC cap.
• 13.5k
Accepted

### Can a small signal BJT handle small currents of reverse voltage (Vce) for a short time?

D16 prevents any reverse current through the emitter. As a result, Q2 essentially becomes a plain diode from collector to base during this reverse discharging operation. This is completely fine and ...
• 16.8k
1 vote
Accepted

### Anyone know what this capacitor is for in a BJT audio amplifier?

C46 is small. It limits open loop high frequency response .If omitted you could get instability. Sometimes this cap is called a compensation cap .Most IC opamps have this done internally.
• 13.5k
1 vote

### BJT circuits under biasing

Yes, you need Beta. The key to this configuration is that a larger Beta (or increased Beta due to temperature) produces a larger emitter voltage and a smaller collector voltage due to increased ...
• 683
Accepted

### BJT circuits under biasing

Applying KVL got me this: $$I_E(R_E + \frac{R_B}{1+\beta} + R_C) = 9.3$$ If $R_B$ were comparable with $R_E$, then the term $\frac{R_B}{1+\beta}$ would become negligible. However, that's not ...
• 16.6k

### BJT circuits under biasing

When DC bias is realized with a voltage divider allowing a divider current which is approximately 10 times larger than the base current Ib we can neglect Ib. (Remember, this is the reason we are using ...
• 23.9k

### BJT circuits under biasing

Yes, you need beta to solve or the other possibility is that questioner assumes beta is high enough that IB can be negligible.
• 1,063
Accepted

### NAND gate with two vs. one BJT transistor(s)

Practical monolithic 1960s RTL NOR gates used two transistors rather than one (eg. Fairchild $\mu$L914). You have to distinguish between something that might work in a given special situation and a ...
• 360k

### NAND gate with two vs. one BJT transistor(s)

Transistors are more expensive than resistors, which are often straight-up free, so if this was a circuit you were designing and having assembled, that would be a big reason to avoid it. I don't ...
• 322

### Base current in a fixed bias BJT

It doesn't change (much) because hFE is fairly stable with temperature (particularly if you consider it in relation to the initial range of hFE). From here you can see that Ib changes less than 3:1 ...
• 360k
Accepted

### PNP: What happens if the base voltage is higher than the emitter voltage?

The maximum reverse bias on the base-emitter junction is around 6 V for many small signal transistors. You could damage the transistor which may not be obvious at first. Over time, the transistor ...
• 6,129

I mean the collector resistor Rc If the transistor is in saturation, and you increase the value of Rc, the transistor will go "deeper" into saturation. The collector current will definitely ...
• 15.2k

Do you remember the definition for saturation? The npn transistor is in saturation state when the potential of the collector is (wrong:larger) smaller than the potential of the base. That means: The ...
• 23.9k

### Broken seven input NAND, what did I do wrong? It worked for 2 days

There's really only three possibilities. Two of them mean the Darlington transistor is dead. Too much current through the Darlington transistor, causing it to fail. A high voltage spike caused by ...
• 65.8k
1 vote
Accepted

### What is the Vid of this Differential amplifier?

If Beta of the BJTs is really high such that IC is almost equal to IE, this structure cannot be biased properly because we expect 1mA to flow in each collector resistor when Vi1=Vi2. This means 12V ...
• 1,063

### What is the Vid of this Differential amplifier?

In your question, you are mentioning a resistive element "re". Do you think that re=Re in your task description? This would be wrong. The quantity "re" is a BJT parameter. ...
• 23.9k

### BJT amplifier design

Just some of my initial thoughts. Perhaps they may help: Your specifications appear to hold a fixed $V_{_\text{BE}}$ value. This never happens, in practice. In fact, $V_{_\text{BE}}$ is ...
• 1,389
1 vote
Accepted

### How to calculate overall power efficiency of multi-stage BJT amp

0.2% efficiency is within the realm of reasonable, depending on the output voltage. Lets go through a mental thought process (answers will be very approximate)... What is the DC quiecent current ...
• 6,129

### 36V P-MOS - PWM driver circuit - totem pole - reduce BJT peak power

https://forum.arduino.cc/t/controlling-the-speed-of-a-dc-motor-but-using-pmos/1054948/29 just change the zener to keep the VGS under VGS(max) example: if VGS(max) is 20V, VGS(min) is 10V and your VCC ...

### Overshoot of BJT circuit driving LEDs

The circuit you show works as shown, and it draws roughly the LED current the formulas imply. But it wastes potentially quite a lot energy unnecessarily. To convert PWM into a roughly constant current,...

### Maximum voltage range for input in emitter follower

The problem It is not related to (NPN) transistor - neither that the transistor cannot sink current, nor that the base-emitter junction is "on" at Vs > 10 V, nor that it breaks down when ...
• 11.5k

### Sharing BJT bias resistors

Yes, but maybe; and, you merely drew it incorrectly. Make a common rail, and run a bias resistor to each BJT: simulate this circuit – Schematic created using CircuitLab The "maybe" is:...
• 13.9k
Accepted

### Sharing BJT bias resistors

This would probably have some common mode currents when each amplifier sinks more current or when the input changes. The other problem is you would have to adjust the resistor divider to allow more ...
• 72.5k

### What will happen if we give voltage to a transistor (BJT) across emitter and collector without connecting the base?

There will be some small current from collector to emitter due to leakage. First of all, linear models of a BJT (like hybrid pi model) often include a resistance in parallel with the collector to ...
• 11.4k
1 vote

### Maximum voltage range for input in emitter follower

Edit: It was pointed out that your lower potential is -10V, and everything I've written assumes 0V! Sorry about that. The principles are sound, though. The only useful working input range for this ...
• 16.6k
Accepted

### Maximum voltage range for input in emitter follower

The problem is that NPN transistor can only source ( injects) current into the node. Try to calculate the current direction at the VE node when VE voltage is equal to -6V. simulate this circuit &...
• 12.9k
Accepted

### Why do we use open loop voltage gain in Miller's theorem? Shouldn't we use the closed loop voltage gain?

To use Miller's Theorem, we must use the actual V2/V1 ratio. If there is local feedback around the transistor, then this must be taken into account. However, CB feedback is in parallel to the ...
• 153k
Accepted

### Optimising standby current of battery operated designs with momentary power activation - circuit example from digital level

For connecting EN to the micro-controller, I tried putting together 2 schemes. Basically we need to create a replica of the EN signal. One going to this power latch and other going to the micro-...
• 1,063
1 vote

### Power MOSFET vs power BJT (Darlington), which has less noise?

You need to distinuguish between noise within the loop bandwidth of your opamp controller, and noise outside. Within the loop, the noise should be dominated by the opamp. Outside the loop bandwidth, ...
• 153k

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