# Tag Info

Accepted

### What causes signal distortion in the BJT circuit I designed?

Your biasing is too close to ground. Here is your circuit simulate this circuit – Schematic created using CircuitLab Here is a simulation when Vbias is 1 V vs when Vbias is 6 V. What happens ...

### What causes signal distortion in the BJT circuit I designed?

You are showing us an emitter follower (or common collector) topology. You've included a DC bias to your base source, which is good to do. But the problem is that while $Q_1$ can source current to ...
• 9,873

### How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

the relation Ic = β x Ib would lead me to believe that the collector current would remain the same, but this obviously cannot be the case. What am I not understanding? Consider the standard ...
• 460k

### Design of a BJT Translinear-Based Square-Rooting Circuit

I cannot figure out how to sink two currents independently from I3 and I4 without having a influence BJT's bias. Current sources do the job. Let's say we scale everything to 1mA: simulate this ...

### Collector and Emitter of BJT are always shorted

Change the transistor to a PNP type in a common-base configuration. Collector to the two 1K parts, base to GND, input drives the emitter through a resistor. Adjust the values of R2 and R3 for the ...
• 20.7k

### Breaking "rule of thumb" for common emitter?

While your actual calculations are slightly off, your general conclusion is correct, that this illustrated stage is not following the 10x Ib rule of thumb. Rules of thumb are just suggestions. If you ...
• 168k

### How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

This question is conceptual, and to answer it, you do not need to know the internals of the transistor, but rather its behavior. When Rc changes In the common-emitter configuration, think of the ...
• 17.1k

### Breaking "rule of thumb" for common emitter?

Typical base current will be around 1uA (hFE = 400) with maximum (worst-case hFE = 120) less than 2.5uA. With a BC239C it will be lower (hFE 380 to 800 with 500 typical) The current through the ...
• 402k

### The LED that is series with relay is destroyed after a couple of activations

You have a 1N4007 diode across each relay coil to clamp the back emf. This diode has a response time, which is slower when it was previously strongly reverse-biased as these are. So the 1N4007 will ...
• 23.2k

### Collector and Emitter of BJT are always shorted

You can do this with two transistors, an NPN followed by a PNP, assuming you have +5 V available (since you're feeding it 5 V pulses this seems reasonable). Here's an example circuit: It's shown with ...
• 23.8k

### How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

The following is a very brief description of how the bipolar transistor works: A voltage of about Vbe=0.7 volts (npn case) effective between base and emitter further reduces the width of the ...
• 27k

### On the huge error between LTSPICE and Ebers-Moll when vCE=0 for a BJT

I think the problem comes from the way your circuit is connected and simplified: (1) You connect the power supply directly to VBE and VCE to GND. This is fine for simulation, but please don't do it ...
• 41

### Is LTspice AC analysis trustworthy at RF when using low impedance sources?

is ltspice AC analysis trustworthy at RF when using low impedance sources? The accuracy of an LTSpice analysis depends entirely on the accuracy of the model you provide it. Did you include all ...
• 130k

### Voltage Controlled Amplifier using Oscillator

The site you link to is about a Variable Gain Amplifier (VGA), but what you want is a Voltage Controlled Amplifier (VCA). https://en.wikipedia.org/wiki/Variable-gain_amplifier This is a more difficult ...
• 20.7k

### Voltage Controlled Amplifier using Oscillator

Forget it. The shown voltage controlled amp provides only 2 gain levels. Switching between them happens by switching the 10k resistor R2 to increase the gain when transistor Q1 is ON. Trying to drive ...
1 vote

### BJT circuit questions

The 100 kΩ resistor provides some hysteresis to the circuit. It forms a potential divider with the 2k2 resistor. As VIN decreases Q1 turns off allowing the 470 Ω resistor to turn on Q2. This has the ...
• 177k
1 vote

### BJT not switching properly

If D13 zener voltage is less than VDD_IN, then R8 will turn on Q10 and a very large current will from from VDD_IN through D13 and Q10. This will cause heating in D13 an Q10, which may damage one or ...
• 4,134
1 vote

### Practical Implementation of A Two Quadrant Transconductance Analogue Multiplier

Connecting Q10's base to Vx is not going to work. It must be connected to Vy. Additionally, the emitter resistor of Q10 must be connected to a significantly lower voltage than the base of the ...
• 19.6k
1 vote

### Why don't these resistors affect the gain in voltage divider bias?

The gain is Vout/Vin. In the small-signal sense, R1 & R2 are in parallel with each other and an ideal voltage source. Since the voltage source has zero output impedance, nothing in parallel with ...
• 24k
1 vote

### How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

Many physical "laws" are approximations which don't precisely how describe how things behave, but provide a practical way of estimating things that would be hard to model precisely. By way ...
• 46.8k
1 vote

### How is a BJT in the active region able to vary collector current without depending on the collector and emitter resistances?

Andy's answer is much more comprehensive than mine here, but a simple demonstration by simulation will no doubt help. Here I connect a voltage source directly between emitter and collector, and I set ...
• 36.5k
1 vote

### Need help debugging a Colpitts oscillator

BJT is biased at 5mA and Collector voltage at 6V. Just by looking at the 16.6k/7.32k divider for the transistor base bias voltage, that is already not the case. At least a 100nF decoupling capacitor ...
• 501

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