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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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is it possible to boost up the current by keeping the voltage constant?

I have a source that can deliver 20mA and 4.5V (MAXIMUM). I need to boost the current to 1 A without using any further source. Also, the voltage should be maintained constant at 4.5V. is that ...
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67 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 ...
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RC integrator+ BJT emitter follower

i am designing a circuit able to draw IV curves of LEDS and diodes through Arduino. Basically i use the PWM function and an RC integrator to have an approximately constant voltage. Then i would ...
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2answers
138 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}\$ (...
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105 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 ...
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1answer
105 views

Input resistance of a BJT common-emitter amplifer

What's the input resistance of the circuit below? I think it is \$R_{in}=R_B || r_\pi\$, but I am not sure
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1answer
122 views

Output resistance of emitter follower voltage regulator

How to calculate an output resistance of such circuit?
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1answer
529 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 ...
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1answer
151 views

Emitter Follower Power Stage of an Amplifier with Switch-off capacitor

I know exactly how emitter follower (EF) arrangement of transistors works (when used as power current amplifying stage). But according to my book (G. Randy Slone: High Power Audio Amplifier ...
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1answer
356 views

Output impedance seen by the load in this emitter follower

I'm trying to understand what the concept of "output impedance" relates in a simple buffer such as an emitter follower conceptually without digging in formulas. Imagine the Rload is about to be ...
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5answers
339 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 ...
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1k views
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1answer
556 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 ...
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1answer
56 views

An emitter follower to isolate MCU from load?

I am trying to drive an LED strip (10 LEDs) from the output of an ESP8266 GPIO pin. I measured the short circuit current from the GPIO and got around 40 mA. My question: Is it recommended to include ...
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1answer
60 views

Emitter follower augmented input current

Hello, I have this emitter follower augmented ( I think that's his name in english) but I don't know from where is the beta+1 on the Iin ecuation?
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1answer
115 views

What would happen to the DC bias voltage of the emitter if this capacitor is omitted?

I'm not quite sure what the question means. I need to explain what would happen if I removed the \$1.0 \mu F\$ blocking capacitor from this circuit. Howver, I'm not sure what the "DC bias voltage of ...
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4answers
685 views

Linear regulator using emitter follower and operational amplifier

I have understood that you can create a linear regulator using a high-current BJT power transistor in the emitter follower configuration and one operational amplifier (and a voltage reference, which ...
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2answers
618 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 ...
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1answer
87 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 ...
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1answer
107 views

Emitter follower with extra load in emitter path

I was reading through Linear Technology's AN70 application note and on Figure 42 there are two parts of the circuitry that I can't understand. It looks like an emitter follower but there is a ...
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947 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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1k 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 ...
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1answer
67 views

Are there methods to stabilizing \$V_{BE}\$ [closed]

For \$V_{IN}\$ >> \$V_{BE}\$ (\$V_{IN}\$ in reference to GND), the \$V_{BE}\$ is quite stable typically only varying in 8mV for silicon NPNs and gets smaller when the emitter is conducting less ...
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3answers
614 views

The need for a current source in an emitter follower

I just want to know what is the need for adding a current source (IE) at the output of an emitter follower topology at the emitter leg of the transistor? Thanks in advance.
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Horowitz lab 4.4

I have a problem doing lab 4.4 from Horowitz's student manual for the Art of Electronics. Here's the circuit and here's what I get from LTspice simulation, which corresponds to hardware circuit ...
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4answers
1k views

Why does the emitter follower clip?

The emitter follower bellow cannot follow the base voltage Vb after some point: Vin input is DC sweeping from +10V down to -10V. The output voltage Vout however stops following Vb at some particular ...
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2answers
2k views

Emitter-Follower Base Input Impedance

I am trying to solve this circuit and am getting confused by how the input impedance of the base of the transistor is calculated. I am confused how this is done? What are the general rules for ...
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3answers
351 views

Current source design

Please help me to calculate the value of resistor R1 so that the current through R2 was 100uA. The only thing I noticed is that the output current is pretty much sensitive to changes of saturation ...
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3answers
1k 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 ...
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3answers
896 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 ...
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1answer
549 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 ...
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1answer
348 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 ...
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3answers
378 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
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485 views

A question on emitter follower use

A DAQ board is outputting 0-10VDC analog voltage via its DAC. The DAC cannot supply more than 5mA so I'm planning to use the following emitter follower for more current(x-axis is the DAC outpout ...
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2answers
101 views

emitter follower output current

Is the output current of an emitter follower out of phase with input current? In the book Electronic devices and Circuit Theory, the current gain, Ai, is negative. It depends on direction on output ...
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3answers
242 views

Why a DC coupled sine input signal will make my biased emitter follower looks like unbiased in LTspice?

I'm trying to simulate a biased emitter follower in LTspice. The dc operating point seems to be wrong once I used a DC coupled sine input signal(zero offset, 5V amplitude, 60Hz), it is almost 0V at ...
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2answers
2k views

How to choose voltage divider resistors for correct biasing in emitter follower circuit?

One simple rule are presented in many sources that I read (for example, Art of Electronics). This rule tells me that I have to choose resistors (R1 and R2) to meet the condition: (R1 || R2) << ...
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2answers
2k 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 ...
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1answer
81 views

Emitter Amplifier Losing Gain

I am designing a color organ and am having trouble with my pre-amp stage. My circuit so far: For low frequencies, my system works pretty well. My input voltage is about 200mv pk-pk, and my output ...
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4answers
2k 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 ...
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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 ...
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1answer
612 views

op amp emitter follower buffer offset

Hi, I'm using an emitter follower op-amp buffer with LM6132 and PN2222A. I'm expecting almost an exact follow?, as simulated with multisim. However, I'm getting an output offset of around 50mV at zero ...
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2answers
273 views

emitter follower conofiguration

I need help with the following configuration , using LIRC for as the source signal for infrared led transmitter circuit with raspberry pi . when putting the infrared led emitter led above the ...
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2answers
582 views

Difficulty in calculating output impedance for the emitter follower

This is the pic for the emitter follower using BJT. Is there a direct way of calculating output impedance instead of the way proceeded in the text book? Textbook procedure: Ps: The images are of >...
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1answer
3k 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?
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2answers
481 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) ...
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2answers
1k 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 ...
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2answers
3k 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 ...
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2answers
332 views

audio blend circuit / emitter follower

This diagram is found online. My input would be audio signal in 0-3.3v range. Will it work under 9v with 2N3904 or BC54x transistors? Does it need component value adjustment while operating under ...
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1answer
988 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 ...