# Questions tagged [base]

A BJT (bipolar transistor) has three leads: base, collector and emitter. In most cases base is used to control the transistor.

76 questions
Filter by
Sorted by
Tagged with
27 views

### Falstad simulator AC vs DC voltage problem

I have a transistor amplifier circuit, which can be found in the included image, as well as a save file text. The problem is, if I apply a 1V AC voltage to the transistor base (of course, with a diode ...
116 views

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

I am very new to transistors unfortunately, so sorry if I don't know every formula already. The thing is, I would like to use a bipolar NPN transistor as an amplifier. It has a current gain of 200. ...
227 views

### I believe the graph in this textbook relating emitter current and base-emitter voltage for a BJT is incorrect. Am I mistaken?

Look at this image relating emitter current and base-emitter voltage for a BJT from Bolyestad. I believe this in incorrect based off this equation for emitter current from Sedra Smith: From this ...
1 vote
161 views

### Transistor switching speed

I want to be able to identify the speed of a transistor in terms of switching. Most datasheets don't indicate that but I found this transistor from ST-Electronics. Is it the maximum switching ...
87 views

### How much current will a BJT transistor draw through its base?

If a BJT transistor has a voltage Vb at its base, its nominal emitter voltage Ve will be Ve = Vb - 0.7. If we know Re, we know the current through the emitter Ie = (Vb - 0.7)/Re. If beta is high, ...
• 1,307
109 views

### Do transistors "prefer" to pull current from their base or collector?

The circuit below is a simple BJT amplifier which behaves exactly as expected: If I remove the capacitive coupling and voltage divider bias and replace it with an input offset, it completely breaks ...
• 1,307
232 views

### Base resistor on BC817-40

There are base resistor calculators and multiple questions are answered on the subject of base resistor calculation. However, the BC817-40 I am using from MCC is showing two different gains (HFE1 and ...
• 55
260 views

### Maximum voltage applied to the BJT base

I have a functional circuit designed for return line reverse protection, and a relevant portion of the circuit is shown below: simulate this circuit – Schematic created using CircuitLab In this ...
• 534
217 views

### Darlington pair problem

I was making a circuit using a Darlington pair with two C3266 transistors and I noticed that the total gain doesn't amount to the expected value. Instead of the total value be in accordance to the ...
• 339
1 vote
148 views

### NPN Transistor Base Current Calculation from Beta, Vth, and Rth

In this answered question regarding a common emitter NPN Transistor.... NPN Transistor common emitter calculation I am struggling to understand the section "Assuming Active Mode" ...
181 views

### DIY Voltage and current regulator

For DIY sake. I'm working on a current and voltage regulator and i found this Online. But instead of a c5200, I'm using the d716 for it. I have a dynamo with 20 volt and 200ma current supply.Is it ...
• 339
33 views

### Stability factor and stiffness of voltage divider on a transitor

Does stability factor and stiffness of the bias network have a connection? this site mentioned something about stability factor so i was wondering if its thesame thumb rule about the current in ...
• 339
57 views

### Biasing using output charesteristics of a transistor

So this is output charesteristics of a transistor (ktc 3198)with variying base current values. Is it okay to design a voltage divider bias with output charesteristics as reference?
• 339
879 views

### Vb and Vbe of a transitor

Is VB and VBE, of a transistor the same? If not, how is Vbe generated? ....and must the base voltage of a transistor be equal to 0.7 or can it exceed this value.
• 339
160 views

### NPN transistor won't switch off in boost circuit

I'm designing a boost circuit to generate ~150V from a 1-4 cell lithium battery. It doesn't need to supply a lot of current; I just need high voltage. Using a 4 cell battery with full charge (16.8V), ...
• 31
34 views

### BJT DC biasing and amplification

Does this part of the datasheet has to do with biasing and amplifying DC using BJT (TIP120)? [!tip120 datasheet]2
• 339
698 views

### What essentially happens when you increase the Base Current in a NPN Transistor?

From what I've studied and read, it doesn't really clear my concept as to why the collector current increases exponentially, more specifically Beta times the base current, when increased. From my ...
• 23
194 views

### Why is it okay to connect the base of CNY17-1 to the gnd with a collector?

Why is it okay to connect the base of CNY17-1 to the gnd with a collector?
• 1
126 views

### Transistor as switch that can handle a high base current [closed]

Which cheap, simple transistor can handle a high base current and doesn't need a base resistor to use it as switch?
• 399
245 views

### How to keep a transistor's base current constant with varying input voltage?

The problem: In the circuit displayed below, I have a microcontroller supplying current to the base of a NPN 2N2222 transistor. When the battery is at 9V, this setup works great: the microcontroller ...
207 views

### Voltage follower simulation low base current

I am trying to get a deeper understanding of how transistors actually work so I can design my own circuits around them. Pictured is an LTspice simulation of a simple voltage follower circuit. I do not ...
535 views

### Increasing the forward voltage required of the PNP transistor?

I am a newbie and was reading about the uses of a 555 TIMER. I came across a project to create 40 LED Bicycle Light in Led Circuits. I can understand the diagram and most of the explanation except ...
651 views

### No resistor to limit base current in this BJT application?

A manual fan controller in my PC just 'sploded: fair enough, it was about 10 years old, and driving more fans than appropriate. I got around to fix it, and figured I needed to change the BJT, because ...
• 417
288 views

### BJT small-signal base resistor voltage

simulate this circuit – Schematic created using CircuitLab I am doing a small signal analysis of the following common-emitter BJT stage and I am trying to calculate the voltage across r_π (v_π). ...
• 117
274 views

### Physics of bipolar transistor turn-off

In a class AB follower push pull amplifier output stage* operated open loop and driven from a low impedance source, these are, from top to bottom, the emitter currents of both transistors, their ...
• 80.3k
196 views

### Calculate base current NPN high side transistor

Say I have this configuration: simulate this circuit – Schematic created using CircuitLab I want to calculate the current going to the base of the NPN transistor. Specifically for PMBT3904,215 ...
2k views

### Shorting NPN transistor base to ground

What is the normal convention(safe and reliable), should one connect NPN transistor base to ground or should a resistor be used? My logic tells me that in common emitter mode, shorting base to ground ...
• 181
418 views

### Determine the Base Current Amplification factor using Ltspice

I'm trying to calculate the resistances of my circuit but I'm stuck because I'm missing the B value and don't know how to find it out . I want to calculate the resistors with which I can determine the ...
• 227
191 views

What does this NPN BJT circuit represent, is it a switch or just a PN diode, and what should this plot look like when we sweep the value of Vbe, and why?
328 views

### NPN transistor switch wireless charger module

I am using this module for wireless charging, as a receiver, and i want to switch it on and off, because firstly it consumes about 5mA when idle and secondly i want to power it off when the battery is ...
• 103
110 views

### Problems finding voltage between two points (transistor)

I am having a homework question about transistors where I am asked to find the voltage drop between the base and the collector (Vbc). I drew the circuit in Proteus but I am not sure where to put the ...
1 vote
496 views

### maximum collector-emitter voltage in BJTs

I am reading BC817 datasheet. According to the datasheet, maximum collector-emitter voltage at open base condition is 45 V. In typical applications we don't use open base configuration. My question is ...
• 113
119 views

### Confusion about NPN base connection

I found an over-current protection schematic, but I am confused about how the base of the Q3 is wired. Is it connected to a potentiometer? If so, to what terminal?
• 403
563 views

### Transistor for 5V relay

Following up this question I need a transistor to switch the necessary current through the coil. I have some spare BC817 lying. My digital output pins can switch 5V@20mA. Do I need a base resistor to ...
• 415
5k views

### Capacitor in series with transistor base

I am fairly new to electronics and just had a quick question. I saw the following suggestion for a one shot trigger to wake up an ESP from for example a PIR sensor with one shot trigger and to read ...
• 171
3k views

### Why the base current is zero in B part of this question?

In the B part of this question , i am not getting why the base current is zero. Everything seems to be fine but still not getting ...
140 views

### Where to find details of dimensions and doping of a typical bjt

I have been searching for dimensions and doping levels of base, emitter and collector for a typical bipolar junction transistor presently in use. I have tried searching on internet and seen datasheets ...
• 261
837 views

### PNP Transistor with low base current

I designed a PCB with the following schematics. simulate this circuit – Schematic created using CircuitLab The circuit uses a MAX6369 external watchdog timer which has an open-drain output(...
• 567
809 views

### How to select a diode for a Baker clamp?

What are the requirements when selecting a diode for a Baker clamp circuit? How does the reverse leakage current of the diode affect the circuit? Can a diode with relatively high reverse leakage ...
6k views

### Transistor base/collector voltage

I've read that with an NPN transistor, there is a voltage drop of about 0.6 V between the base and the emitter, so you need to have about 0.7 V on the base to switch it on (circuit 1). But what ...
79 views

### Base resistor configurations, are there any differences?

Sorry for the noob question. From my understanding, R1s acts as pull down resistors for the base and R2 will limit the current flow to the base. I have seen both configurations, but couldn't work out ...
6k views

### Circuit with Base of Transistor connected to ground

I am having troubles understanding a circuit where to base of a transistor is connected to the ground. As far as I know a transistor needs a certain voltage on the base so electricity can flow from ...
• 23
220 views

### maximum emitter base reverse voltage limiting when driving with op amp?

This is part of a larger circuit, but problem area is shown below. In simulations (of the larger actual circuit not shown), the base voltage of Q1 drops to about -6.35V. Can this damage Q1 (2N3904)? ...
• 2,187
344 views

### Calculating base resistor value using voltage

I'm pretty new to electronics so my question is I think a relatively simple one. On transistor datasheets, the base-emitter voltage is typically around 6V, which means that voltage across this ...
1 vote
2k views

### Negative base current

I am designing a flyback driver using a salvaged power transistor that was originally the driver transistor of a flyback in a ctr screen. This power transistor is the 2SC5681 I want to drive this ...
647 views

### I need to calculate the NPN bjt 2N4401 base resistor

I'm using the transistor (NPN bjt 2N4401) as a switch to light up 20 leds (leds use 2v 20mA) (my power source is 6V). How to calculate the base resistor?
963 views

### Is maximum BJT VEBO rating exceeded in this miller effect compensation circuit?

I have an switching circuit containing a transistor to switch the inductive load. There is a miller effect compensation capacitor (C1) in parallel with a base resistor. (The values of the two ...
• 288
1 vote
1k views

### Mixing down to base band

What exactly is the base band? I always thought that it is a synonym to intermediate frequency but it appears not to be the case. For example I have Bluetooth signals in the range of 2.4 to 2.48 GHz. ...
• 171
1k views

### NPN driving PNP, Diode from NPN Base to PNP Collector?

I am reviewing some old 12V alarm designs that use the circuit below to activate a siren. The design has been built on PCB and I have confirmed proper operation, proving the schematic is not flawed (...
• 23