# Tag Info

### I connected two BJT NPN transistors and don't quite understand how it works

There should be some current-limiting resistor in series with the base. If you apply 3.3V the red LED and the transistor base will see a large and poorly defined (and possibly destructive) current. So ...
• 356k

### I connected two BJT NPN transistors and don't quite understand how it works

First, add one thing to your circuit: a series resistor on T1 base (1k ohm or so) to limit the base current when the input is above 2.2V or so. Otherwise the excessive base current will fry both T1 ...
• 46.2k

### BJT: Relation between Ic and Ib in saturation

Definition of saturation: A collector current that produces a collector voltage Vc which is smaller than the base voltage Vb (npn case). That means: Because of Vbc>0 the base-collector junction now ...
• 23.7k
Accepted

### BJT Active low-pass filter frequency calculation

Ah, it's "one of those sites", with their fantastic made-up circuits. The circuit is reproduced below. simulate this circuit – Schematic created using CircuitLab The response from ...

### I connected two BJT NPN transistors and don't quite understand how it works

Step 1: Trying to see the basic idea In the OP's circuit diagram (I have used the @Spehro's schematic as a base), we see that the two LEDs are controlled by two transistor stages - inverting (Q1) and ...
• 10.9k

### How can there even be an input voltage on a long-tailed pair?

Base-Emitter junction is a PN junction, and works like a diode would, absent current gain. So, for an initial analysis, replace the transistors with diodes from base to emitter, and collectors open, ...

### NPN BJT on voltage

Assuming an NTC thermistor in an unbuffered resistor divider, the circuit is more like: simulate this circuit – Schematic created using CircuitLab (Using a PTC thermistor would swap the ...
• 616
Accepted

### I connected two BJT NPN transistors and don't quite understand how it works

Why not using a pnp transistor as in the attached configuration
• 36
1 vote

### Why isn't my output voltage fully amplified?

Your choice of R1 and R2 are trying to place the base at about +4V, which would put the emitter at 3.3V. With 3.3V across about 1K of emitter resistance, you are expecting 3.3mA emitter current, and ...
• 12.7k
1 vote

### I connected two BJT NPN transistors and don't quite understand how it works

For normal NPN transistors, Vce at satuaration is much smaller than Vbe. Thus, when T1 is on, Vce of T1 is smaller than Vbe of T2. T2 remains as off. For example, for the 2N2222: V_{CE(sat)}=0.3~\...
1 vote
Accepted

### BJT characteristic curves, Vce vs Ic

After many tests I have discovered that only the two units I have of the BC550C behave in this strange way I have subsequently tested with PN2222A and BC337 and both behave correctly as indicated by ...
• 225
1 vote

### Gain loss in two-stage direct-coupled amplifier

From what I see Ic1 is around 4mA and Ic2 around 45mA. Thus, $r_{e1} = \frac{26mV}{4mA} = 6.5\Omega$ and $r_{e2} = \frac{26mV}{45mA} = 0.58\Omega$ So the voltage gain of the first stage is around ...
• 12.8k
1 vote
Accepted

### Is this text wrong about output impedance variation

You're right. It seems the author confused himself. The higher the slope, the smaller the output impedance. So it should read ... resulting in a decrease in output impedance. Is this the Sarma book, ...
• 2,121
1 vote

### BJT: Relation between Ic and Ib in saturation

This is from onsemi's BC547 datasheet: The active region corresponds to the vertical part of these curves. In this region, for a constant collector current, the base current also is (almost) constant....
• 16.3k
1 vote

### How can there even be an input voltage on a long-tailed pair?

The voltage across the base-emitter junction of a BJT is constant in forward-active and saturation mode. The trick is that the base-emitter voltage is not absolutely constant but varies within small ...
• 10.9k

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