# Questions tagged [common-base]

A common base is one of three basic bipolar junction transistor (BJT) amplifier topologies. It has a low input impedance, produces a high voltage gain, but its current gain is less than 1. Its high-frequency characteristics are better than of the other two BJT amplifier topologies.

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### Why does the LC oscillator work according to the scheme with a common base?

I study HF oscillators and i stuck. I put build the following circuit: The waveform: A question arises. Why does this work at all, because the input resistance of the circuit is extremely low. In ...
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### How to analyze common-emitter amplifier by reflecting base impedance to emitter?

In chapter 2 of Dr. Middlebrook's video series Technical Therapy for Analog Circuit Designers he gives 2 examples of how solving a CE amplifier by "doing the algebra on the circuit diagram" ...
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### Does it always have to be I_E = I_B + I_C for a transistor circuit? [closed]

I found this common base configuration of an npn transistor, and here it is - I_B = I_E + I_C, and the circuit seems moving. Is this considered a transistor circuit or not? the video - https://...
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### Can we replace transformer instead of transistor for some particular cases? [closed]

In a transformer, the alternating current and voltage is transformed in the secondary coil by keeping the power constant. When we are dealing with AC, suppose that we want to amplify the voltage ...
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### Why are only 3 of 6 single-transistor amplifier configurations useful?

Almost any analog design book or course I have ever seen starts by introducing the three basic single-transistor stages. For example, in a bipolar technology these are the common-emitter, common-base, ...
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### Determining which resistors to use in a common-base amplifier

The assignment is to design a common-base amplifier from a supplied schematic, and quite frankly amplifiers are a significant weakness of mine. Given: Small-signal voltage gain A_V≥50 V⁄V Input ...
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### Common base configuration

I have tried to implement this circuit of common base configuration but the power supply used in the lab is very confusing and I suspect it’s the reason why my readings aren’t turning out right (it’s ...
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### Does changing the collector resistance of a common base amplifier have any effect on the current?

I have come across this equation for understanding the voltage gain of a common base amplifier: $$A_v = \frac{I_c R_c }{I_e R_e}$$ Because base current is small, Ic and Ie are roughly equal. ...
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### Why does this implemented Hartley oscillator's amplitude vary significantly?

I have implemented the below Hartley oscillator in LTspice for analysis before implementation. Later, it was implemented on a circuit board. I see it oscillating in LTspice at the calculated frequency ...
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### An NPN transistor without its collector-base voltage [closed]

What would happen if I remove the base-collector voltage from an NPN transistor and put a wire connecting two ends of the voltage source? I believe that it it will work nicely. As the collector ...
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### Common base amplifier's input impedance

simulate this circuit – Schematic created using CircuitLab In a BJT common base amplifier configuration the input impedance is ~$r'$ (emitter resistance), right? If I understand correctly, ...
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### Designing a common base amplifier to meet given specifications

I designed the circuit as below and I know that common base amplifier should have a unity current gain(usually <1) but I'm getting a very high gain. What might be the reason for this, is something ...
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### Common base question

I'm trying to solve this problem where I find Vout/Vin if rπ if infinite. I also have to conside early effect. When I try to solve it, I get Vπ = 0 and that makes gm zero, a bit confused on that.
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### Common Base Amplifier Confusion

In the CB configuration, a signal is applied to the emitter to create an in-phase amplification at the collector. I am confused, however, on how exactly this happens. In the picture below, if the ...
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### How did they get the formulas for Av, Rin, and Rout for this problem?

How did they get the formulas for Av, Rin, and Rout for both problems? I understand how they solved it. I just want to know where they got the formulas because I can't seem to find them anywhere. ...
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### Detecting absence of current in transformer secondary

Beginner here. I'm trying to build a circuit which charges a capacitor to an arbitrary voltage in the shortest possible time. I want the high voltage part of the circuit to be galvanically isolated ...
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### Is a special BJT transistor to use in Common Base configuration?

Revisting multistage amplifier consists of Common Source-Common Base-Emitter Follower in cascading, I found the attached page of MPS-H85 PNP transistor that is proposed special for common base ...
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### How to connect the ground in this common base schematic to a breadboard? [duplicate]

I just want to know how do I connect the ground in this common base circuit. I know how to connect two DC sources but I'm confused on how to connect the ground. I have adept knowledge about the ...
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### Transistor Analysis of a Common Base(?) BJT

Hey. I am a biomedical engineering student focusing on the biomaterials side. However, my university makes me take a few electrical engineering classes as well. So I am not very good at this. I was ...
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I have built an FM radio receiver from a kit. I'm very happy with the results! It is the Sinclair Micro FM. My goal is to learn about discrete electronics (NO IC's.) The radio works great, but I ...
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### Is Icbo amplified in reality in Common Base configuration?

Regarding the active mode of operation: Icbo is leakage of the reverse biased C-B junction. It flows whether or not the emitter is floating, as long as there is a complete C-B circuit. The emitter ...
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### Is this way of plotting AC current gain in LTspice correct?

I want to plot the AC current gain versus the collector current in common base configuration. Ac current gain "alpha ac" for CB configuration is defined as the change occurring in collector current IC ...
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### Common base bias divider resistances

I have been trying to grok the common base amplifier, and put together a few examples, but there is something that I do not get at all. I have the setup below. Things are working more or less as ...
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### finding output resistance of CB amplifier with ro

I have this CB amplifier circuit below This is the small signal equivalent circuit arranged to find the output resistance R_out with the r_o included Here is my analysis below to find the output ...
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### Purpose of PNP transistor in ultrasonic driver circuit

This question is related to my other question What is the purpose of Q2, R2, C1 and C6 in the circuit in the original question? When I remove these components from the simulation nothing appears to ...
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### Identifying the configuration of transisitors

I have been handed the diagram below, as part of an introduction to transistors. I am having a hard time finding out what type of configuration it is, therefore making it even harder for me to find ...
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### BJT common-base output resistance derivation

In Sedra/Smith the following image may be founded that is used for output resistance equation derivation of common-base BJT amplifier. The question is why the gm*v and v/re currents are directed up? ...
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### Common base amplifier output impedance

In Sedra Smith I can found that: Rout = ro + (1 + gm * ro) * Re_s, Re_s = Re || r_pi. Can I see in my circuit using simulation tools (like current probe, voltage meter) that previous equation ...
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### Confusion with power amplification concept in common base topology

A text is telling that a transistor can amplify power and gives common base amplifier as an example. Here is what it mentions: But what is the use of the transistor here? Below I draw the same ...
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### Common Base amp gain problem

simulate this circuit – Schematic created using CircuitLab Here's a little schematic I built for my 27MHz colpitts oscillator, which outputs approximately 120mVpp. Since the common base is less ...
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### Collector current decreases with the increase of base current in saturation mode of bjt

I'm new to BJT and i'm trying to understand the characteristics of bjt in different modes of it. In my book they explained that in saturation mode of bjt the collector current decreases even if i ...
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### When two transistors connected which one is on and which one is in cutoff from given circuit condition?

here in this question's solution, given that upper BJT is on while lower one is in cutoff region for Vbb = 2.7 V and viceversa for Vbb = -2.7 V and both will be in cutoff for 0 V. While in other ...
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### Transistor noise figure in common base / common gate configuration

My question regards BJT and JFET devices. Noise figures for transistors are usually given along with specific signal source resistances and collector / drain currents. How can one apply these ...
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### All possible BJT circuits relating to polarity

The BJT can operate in 4 different modes/zones since there are two voltages on it: Active mode Inverse/Reverse active mode Saturation mode Cut-off mode Because Emitter and Collector junctions can ...
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### Strange colpitt's oscillator(s)

I'm currently building a theremin from the circuits form here:LINK This uses Colpitts oscillators. However, when I've tried calculating the frequency, for example of the fixed-pitch oscillator (below),...
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### Why does the emitter current increase with an increase in collector to base voltage in the common base configuration of a BJT?

It is said that because of the Early effect that with an increase in collector to base voltage there is decrease in base width due to which $\alpha$ (the current gain) increases due to lesser ...
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### Common base circuit with zero supply voltage

Consider a bipolar junction transistor (either PNP or NPN). Which output would produce this circuit for various input ($= V_{\mathrm{BE}}$) voltages? It is common base, but without a voltage supply ...
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### Transistor Common Base configuration

It is said that for a common base configuration the base is grounded, i.e. at zero potential. But at forward biased for a npn transistor, the battery used to forward bias emitter–base junction is ...
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