Questions tagged [emitter-follower]

An emitter follower (also known as a common collector amplifier) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.

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17
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6answers
4k views

What does this transistor circuit do?

I found a schematic which had this bit of circuitry between the +9 V (which apparently the author assumed to be > 9 V, more like 10-11 V) barrel connector and the +9 V supply rail. I tried to ...
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5answers
2k views

Art of Electronics: Emitter-Follower Zout

I am a growing frustrated with the Art of Electronics. It is such an approachable book in Chapter 1, and then in Chapter 2 it seems like the authors wanted to make it more textbook-like and they ...
7
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1answer
337 views

How to design a good common-collector amplifier?

I tried to simulate an example common collector amplifier given in the book Electronic Devices by Floyd. The circuit depicted in the book is: I used LTspice with transient analysis to implement and ...
7
votes
1answer
566 views

Down-converting voltage

I'm trying to reduce the voltage of a power supply for a certain load without affecting the current the load draws. Since the op-amps I could find were not able to provide sufficient current, I added ...
7
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2answers
3k views

Opto-isolator output from collector vs. emitter

I have an Isocom ISQ2X quad opto-isolator (datasheet) that I will use to drive a logic input. The opto-isolator's simplified model given by the datasheet is pictured below. The transistor base pin is ...
7
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2answers
770 views

Why the base current decreases automatically?

Given this circuit: simulate this circuit – Schematic created using CircuitLab I don't understand why, when simulating it by increasing parametrially the V2 from 0V to 12V keeping V1 constant, ...
6
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3answers
2k views

Emitter follower regulator with sziklai pair, transistor heating up!

I have a wireless chip esp8266 that requires input voltage of 3.3V and can draw current up to 500mA. Unfontunately at the moment I don't have any 3.3v regulator IC's or buck converters or any 3.3v ...
6
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2answers
5k views

How does this power supply circuit work?

This is the circuit of a power supply with adjustable output voltage and current, and I wish to understand how it works: What I did understand: R1 T1 VD5 - they form a voltage divider which provides ...
6
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1answer
2k views

How to select resistor for emitter follower when driving speaker?

simulate this circuit – Schematic created using CircuitLab The objective is to drive 1 watt into the speaker. R1 represents the output impedance of the previous common collector stage. Here's ...
5
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3answers
4k views

Emitter Follower Output

I cannot afford a function generator and oscope right now, so I'm trying to learn analog electronics with Spice (MacSpice, to be specific). Below is the Spice model for a simple emitter-follower ...
5
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1answer
903 views

Emitter follower

I'm having a hard time understanding an emitter follower. What I don't understand is why there is no voltage gain but only current gain. From my understanding when the transistor is turned on, the CE ...
5
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3answers
14k views

Emitter follower: Icq and Vceq calculation

I have problem with understanding how to calculate \$ I_{cq} \$ and \$ V_{ceq} \$ of this circuit. Q1 has these details: \$ h_{FC}=150 \$ and \$ h_{fc}=100 \$ As I understand it I should find \$ V_{...
5
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6answers
4k views

Designing a stiff voltage source using an emitter follower

simulate this circuit – Schematic created using CircuitLab I am new to electronics, and I am trying to do Exercise 2.2, page 67 of The Art of Electronics. Use a follower with base driven ...
5
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2answers
3k views

Controlling the base of a N-Channel MOSFET on the high side of a circuit

With the help of some other members of the site, I was recently able to construct a gate driver circuit for some MOSFETs. I was operating N-channel MOSFETs on the "low side" of a circuit. Meaning that ...
4
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5answers
8k views

Stiff Voltage Divider Bias for an Emitter Follower

Below is an ac-coupled emitter follower with base bias provided by a voltage divider. I'm having a slight issue with how the resistor values are chosen for the biasing in the design example provided ...
4
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3answers
2k views

The need for a current source in an emitter follower

What is the need for adding a current source (\$I_\text{E}\$) at the output of an emitter follower topology at the emitter leg of the transistor?
4
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1answer
87 views

Are base resistors of any use for a high-side NPN switch?

I "inherited" a small project from a person I don't know, and I'm navigating my way through it. It's a small gadget featuring a 7-segment LED display. I hear the device's original designer is more of ...
3
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3answers
218 views

Op amp Power supply splitter with totem pole

I want to make a +- 2.5V rail from a 5V supply using an op-amp. I was wondering if I could use a totem pole for driving the output like this: Image from here But while I was looking at it, I noticed ...
3
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2answers
380 views

How improve temperature sensitivity of HP 721A DC Power Supply voltage reference?

I have a sentimental affection for my HP 721A DC Power Supply: http://hpmemoryproject.org/pict/wall_a/anim/721a_q90/viewer.htm Rugged, dependable, and roughly as old as I am, it is also pretty ...
3
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3answers
1k views

Confusion on the meaning of a BJT emitter follower's output impedance

Above figure is a BJT transistor emitter follower. I can understand that in an emitter follower when talking about its input impedance what meant is Vb/Ib and here it is equal to (hfe+1)*R. The idea ...
3
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2answers
349 views

Emitter-coupled logic operation

For the operation of emitter-coupled logic and sedra smith book, 1) Compared to CMOS, why ECL is the faster logic family given that it requires so many transistors to propagate to output Y (T5 ...
3
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1answer
587 views

Horowitz's exercise 2.2

Having done the exercise 2.2 from Horowitz's "The Art of Electronics," I feel uncertain and would like somebody familiar with the matter to check my solution. With this aim in head I've searched the ...
3
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2answers
751 views

Advantage of using an op-amp connected emitter follower for LED driving?

I came across the following circuit used to set a microcontroller-controlled fixed current in an LED to around 100mA (target current): As far as I understand, the voltage reference at the IN+ (1.2V) ...
3
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1answer
291 views

Inconsistency in the calculation of the gain of an emitter follower

I want to calculate the gain of the following circuit, but two different ways give different results. simulate this circuit – Schematic created using CircuitLab First method: (This method is ...
3
votes
1answer
85 views

How is the quiescent current and emmiter resistor determined on the driver and pre-drivers of a class-AB audio amplifier output stage

I am reading online about discrete audio amplifiers. I have learned about the input differential pair, the voltage amplifier stage, and now I'm learning about the output stage. I have learned that you ...
2
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5answers
352 views

(logic level) Gating the output of a 900hz (5v p2p) oscillator

Seems a simple enough problem this, and perhaps all that is needed is confirmation of approach (although a better solution always appreciated). So the problem: I have a 555 based square wave based ...
2
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4answers
4k views

Why emitter follower is so noisy and distorted in large signal?

I have a signal around 20 V peak-peak. I want to feed it into an emitter follower BJT and I have no experience in working with such large signals. The BJT is well in bias point ( I tested the ...
2
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3answers
837 views

BJT switch vs amplifier mode

I'm getting really confused about a specific BJT circuit. I want to control LCD brightness and contrast as suggested here. Apprently a capacitor is used to filter PWM output to the contrast pin, and ...
2
votes
1answer
332 views

Why do I have clipping in this emitter follower

I am trying to design a class A amplifier buffer stage (I know it is not efficient.) I am starting from the simple emitter follower and the next step is to replace the RE resistor with a current ...
2
votes
2answers
871 views

Emitter follower voltage regulator misbehaving

To make a crude voltage regulator, I've made a simple emitter follower using a BD139 transistor. When the 0.5A load is connected, the circuit behaves as expected: the voltage across the load sits at ...
2
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2answers
4k views

The advantage of using base biasing over emitter biasing?

Can someone please explain to me what is the advantage of using base biasing over emitter biasing. Why is a 'feedback resistor' used in base biasing?
2
votes
3answers
813 views

Square wave into emitter follower

I put a square wave of relatively low rise and fall time (of about 10ns), and wanted to see the response from an emitter follower output. I put together the circuit and here's what I see on scope. I ...
2
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3answers
880 views

Loading effects of a signal source

I'm reading "The Art of Electronics" (3rd edition) by Paul Horowitz and Winfield Hill. In the 2nd chapter on BJT (page No. 79, section Emitter follower). The author says: ".. In general, the ...
2
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3answers
568 views

Voltage gain of an Emitter Follower

Please see attached image. Can you please help me understand how they went from the first to the second? Not too sure how they got (B+1) in there. Thanks
2
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1answer
82 views

Emitter-Follower Class A amplifier

I was trying to solve a problem from SEDRA-SMITH book. I got stuck so I refer to the solution. When Studying how the author solved the problem I got even more confused. Let's refer first to the ...
2
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3answers
1k views

Emitter Follower LED Circuit - LED in Collector, Resistor in Emitter

I have seen some examples of using an emitter follower circuit with an NPN transistor to light an LED. In those examples, it's mentioned the collector voltage must be equal-to or higher than the base ...
2
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1answer
1k views

How to choose the emitter resistor for a common collector configuration?

This link feature a video starting at a specific time: https://youtu.be/t0UOSIUve9E?t=16m24s For a common collector configuration, The teacher chooses the emitter voltage (output voltage) to be the ...
2
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1answer
166 views

Is an op amp voltage follower better than one made with discrete (MOS)FETs?

Is there any advantage of the following op amp based voltage follower:- over one made with discrete MOSFET components such as:- other than component count?
2
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1answer
99 views

Using BJT emitter follower after microcontroller

I am trying to understand an already existing circuit shown below taken from http://boris0.blogspot.fr/2013/09/rotary-dial-for-digital-age.html. The aim of this circuit for the right part is to ...
2
votes
1answer
677 views

Op amp voltage reference with emitter follower

The purpose of this circuit (taken from the Minimoog) is to provide a stable -5V across the 100R load resistor, as accurate as the -10V supply and the ratio of the 5K resistors in the potential ...
2
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2answers
556 views

Emitter follower and input impedance

I'm designing an active bandpass filter (using only BJTs, voltage sources, resistors and capacitors) with the following properties: Corner frequencies \$8 ~ \text{kHz}\$ and \$800 ~ \text{kHz}\$ (...
1
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4answers
254 views

Emitter current

Suppose when the collector is connected we get \$I_b=10 mA\$, and \$I_c=1000mA\$, so naturally we have \$I_E=1010mA\$. What will happen when the collector is disconnected? Will the base current change ...
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2answers
534 views

Why is the base resistance suggested to be one-tenth of the emitter resistance times beta?

This is a follow up question from Emitter follower biasing with voltage divider or the emitter follower design example in The Art of Electronics. Why is the recommended one-tenth ratio of base ...
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2answers
2k views

Emitter follower biasing with voltage divider

I have met a emitter follower design example in the Horowitz-Hill's book (scheme is below): And I don't understand why the resulting steps is correct: Step 1. Choose \$V_E\$. For the largest ...
1
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4answers
4k views

A question on voltage divider resistances used in BJT biasing

I'm reading a text on BJT common collector amplifier biasing. Text says: The voltage divider (R1 and R2) is used to give the input signal (after passing through the capacitor) a positive dc level ...
1
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1answer
186 views

Would you help me understand this simple circuit?

I have three questions: Is it common emitter or common collector (emitter follower) or something else? How do you recognize it? What is the function/usage of R1 and C1? Are they for biasing the ...
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2answers
107 views

Emitter Follower as a Current Amplifier

Reviewing Basic amplifier stage properties, I found unanimously that a Current Amplifier has a Very Low (ideally zero) Input Impedance and a Very High (ideally infinite) Output Impedance. The Emitter ...
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2answers
1k views

Single supply op-amp voltage follower - negative clipping with AC-coupled load

I am using a µC running DDS algorithm to synthesize certain waveforms (in fact I made 3 channel music generator with portamento&ampltude modulation effects, commands for changing waveforms etc. ...
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2answers
884 views

Voltage Follower Saturation

I'm trying to use a 0.5V signal as a source. I've wired a TL072CN as follow: VCC+ to 11V VCC- to GND OUT2 to IN2- IN2+ to 0.5V The thing is when I measure the voltage on OUT2 I get 10V where I am ...
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1answer
58 views

Where voltage is lost in an emitter follower?

I'd wanted to know what happens in an NPN emitter follower with \${\delta}V=Vcc-Vee\$. At which junction does voltage drop? May this drop be fatal to a transistor?