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66 votes
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What's the purpose of these two diodes in this circuit?

This is a crude current source. The two diodes are to create a voltage drop of approximately 1.4v, which is then applied across the transistor VBE and R21. This gives approximately 0.7v on R21, or ...
Neil_UK's user avatar
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56 votes
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Why use two transistors driving LEDs when one would work?

It's a constant-current circuit. The current should be about 0.7/68 ~=10mA. simulate this circuit – Schematic created using CircuitLab Your circuit with one transistor would allow a highly ...
Spehro Pefhany's user avatar
25 votes
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PNP high-side transistor switching with microcontroller

You are trying to create a high-side switch. simulate this circuit – Schematic created using CircuitLab Figure 1. (a) NPN open collector switch. (b) A failed attempt to make a PNP version. ...
Transistor's user avatar
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22 votes

Why use two transistors driving LEDs when one would work?

The circuit with two transistors, even if drawn in a non-obvious way, is a constant current circuit for driving the LED. When enough current flows via 68R resistor, the voltage over it rises, which in ...
Justme's user avatar
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20 votes
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Can someone explain this microcontroller ADC interface (for reading solar panel voltage)?

What is the role of the pnp transistor network? It's a differential voltage to current converter followed by a load (R34 and R35). The voltage between P+ and P- sets a voltage across R31. This (minus ...
Andy aka's user avatar
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20 votes
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What are the tradeoffs between NPN vs PNP power transistors?

Without fully exploring the topic (which could go on forever), just some points to highlight: An external-transistor regulator like '723 may have lower voltage drop with PNP. Notice, as long as base ...
Tim Williams's user avatar
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18 votes

What's the purpose of these two diodes in this circuit?

Both diodes together produce a volt drop of about 1.4 volts, hence the base is at a voltage of (5 - 1.4) volts. This means the emitter is 0.7 volts higher (due to the base-emitter region being a ...
Andy aka's user avatar
  • 452k
18 votes

Is it possible to make a high-side PNP switch circuit active-low with less than 3 BJTs?

Original answer Okay. I gather you can sink a lot of current with your I/O pin: about \$25\:\text{mA}\$. But you can source only: about \$170\:\mu\text{A}\$. I don't really care that much about ...
jonk's user avatar
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17 votes
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Diode connected BJT instead of a diode

"Reinforced diode". Diode connected BJT aka "active diode" is simply a transistor which collector is connected to the base. Thus the collector-emitter part of the transistor is ...
Circuit fantasist's user avatar
17 votes

How to determine whether a given transistor is NPN or PNP?

If you know the component is a BJT transistor, you can use a diode tester and test the pins pairwise. If you find two diodes with a common anode, and no conduction between the two cathodes, then you ...
user2233709's user avatar
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15 votes
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Why NPN and PNP in parallel in this distortion pedal circuit?

Because both transistors are biased into their linear regions all the time, this circuit is not a "hard" clipper such as two inverse-parallel diodes, a transistor that goes into saturation ...
AnalogKid's user avatar
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15 votes
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Can different manufacturers have same part number for a transistor?

Manufacturers, in general, can use whatever name they want, even if the characteristics or even functions vary. If they are members of some organization such as JEDEC they may have agreed upon some ...
Spehro Pefhany's user avatar
13 votes

How to distinguish PNP and NPN transistors?

This is pretty easy. I used to do this routinely in high school when salvaging parts from unknown discarded boards. As Spehro said in a comment, sometimes you can find a part number. In that case, ...
Olin Lathrop's user avatar
13 votes
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Control 5 V load with transistor 3.3 V - Raspberry Pi

Figure 1. In this example Vss is greater than the 5 V supply of the micro-controller. The protection diodes keep the transistor always on. Figure 1 shows the internal schematic of a 5 V powered GPIO ...
Transistor's user avatar
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12 votes
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How can I make my PNP amplifier circuit to turn off faster?

The basic idea is something like this: simulate this circuit – Schematic created using CircuitLab There are problems with this, though. For example, \$Q_3\$ might oscillate. Some added base ...
jonk's user avatar
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12 votes

Diode connected BJT instead of a diode

A diode connected BJT has much better ideality factor than a regular diode and is used where close to ideal behaviour is required, such as in silicon temperature sensors. Many of these sensors ...
Peter Smith's user avatar
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12 votes
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How does this garden solar light work?

I do not understand how the small current from emitter to base makes it back to the negative terminal of the battery, how is it getting to ground? Individual PV cell is like a diode so it can ...
kaosad's user avatar
  • 586
11 votes

What are the tradeoffs between NPN vs PNP power transistors?

Some circuits are just more convenient with PNP due to where the transistor is required to be in the circuit. It is usually simplest to provide a base signal to control BJT relative to one of the ...
DKNguyen's user avatar
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10 votes
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Discrete negative voltage regulator oscillating under load

It's not oscillating What you are seeing is ripple from C1 causing excessive variation of the current through D1 and varying its voltage. That voltage then directly affects the output voltage. You ...
Kevin White's user avatar
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9 votes
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Does a PNP transistor pull up itself?

That's actually pretty easy. You have a schematic editor you could use here. I'll use it now. I've paired them, side by side, to match up with your pictures in your question: simulate this circuit &...
jonk's user avatar
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9 votes
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Understanding a PNP/NPN LED schematic

Overview Just to get the schematic into the editor supported here and to draw it in a slightly better form for analysis: simulate this circuit – Schematic created using CircuitLab I've dumped ...
jonk's user avatar
  • 77.8k
9 votes

Diode connected BJT instead of a diode

Usually in very low leakage applications, e.g. high impedance input clamps to power rails, or log amplifiers, where leakage would disrupt the current to voltage relationship your hoping to make use of ...
Reroute's user avatar
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9 votes

How to determine whether a given transistor is NPN or PNP?

The method dscribed by @user2233709 works to find the base and to determine if the device is PNP or NPN. Which is C (collector) and which is E (Emitter) and a very rough indication of Beta can be ...
Russell McMahon's user avatar
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9 votes
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PNP current flowing in reverse direction

A PNP transistor (or NPN) can work in reverse. The gain (β) won't be so high but, it'll work in this circuit. The device is PNP and ostensibly has a symmetrical topology. OK, the doping is different ...
Andy aka's user avatar
  • 452k
8 votes

8051 - Can an NPN transistor be driven?

suspect ... not sourcing current, they are sinking Don't guess or go by heresay, READ THE DATASHEET. That will tell you outright how much current particular port pins can source. Most likely, most ...
Olin Lathrop's user avatar
8 votes

PNP transistor constant-current source

I would like to know if it is necessary to add a base resistor to the Q1 transistor No. The circuit looks like a 12-to-13 mA constant current source, so D1 has a constant current despite variations ...
AnalogKid's user avatar
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8 votes
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Turn off PNP BJT with high impedance output

Even if you set the pin to high impedance, voltage on it must not exceed the maximum ratings in the datasheet, which is probably close to 3.3V. There could be an exception if the pin is labeled "...
bobflux's user avatar
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8 votes

How does this garden solar light work?

Some circuits should be redrawn to de-clutter: - How does this garden solar light work? When the solar panel illuminates and produces up to 4.5 volts, the 1N914 is in the charge path to the battery ...
Andy aka's user avatar
  • 452k
7 votes
Accepted

Removing offset due to PNP output

You're using the PNP as an emitter follower so no wonder it adds a voltage to the bottom side of the waveform. What you need to do is use an NPN or an NMOS transistor. Then the polarity of the signal ...
Bimpelrekkie's user avatar
  • 80.8k

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