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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1answer
170 views

Discrete component FM radio receiver circuit - explanation needed

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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1answer
41 views

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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1answer
42 views

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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1answer
32 views

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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2answers
205 views

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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1answer
129 views

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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2answers
160 views

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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2answers
650 views

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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1answer
854 views

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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2answers
66 views

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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4answers
2k 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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1answer
525 views

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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3answers
143 views

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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1answer
259 views

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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1answer
212 views

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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2answers
237 views

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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1answer
671 views

Why Emitter current increases with increase in collector to base voltage in common base configuration of a BJT?

It is said from early effect that with increase in collector to base voltage there is decrease in base width due to which \$\alpha\$ (the current gain) increases due to lesser recombination of holes ...
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2answers
508 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 ...
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3answers
306 views

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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2answers
642 views

Amplifier gain too HIGH?

I have this circuit : Two common base amplifiers for amplifying frequency in FM range (88 to 108 Mhz). The Problem is, as seen in the figure, I am getting an output of 1.5 V for an input of 10 uV. ...
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3answers
209 views

Why is KCL giving me the wrong solution?

I'm trying to derive the gain of the following common base amplifier circuit: To do this, I drew the equivalent T-model circuit: . The problem I have is that I get two different transfer functions ...