# 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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34 views

### 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 ...
173 views

### 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 ...
2k views

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 ...
4k views

### 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 ...
1 vote
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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 ...
1 vote
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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 ...
1k views

### 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 ...
1 vote
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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 ...