# Collector current saturation on BJT

I took a look at a few answer such as this and this, where they say the collector saturation current is entirely dependent on an external source. What happens if the source is 1kV or 10kV? doesn't the transistor has its own collector current saturation? For reference, I want to design a NOT gate and need to choose the values for R and R2 (see here).

• The main idea about BJT saturation (not FET) is that the base-collector junction becomes increasingly forward-biased, the collector increasingly "looks like" a voltage source instead of a current source, and the ratio between collector current and base current ($\beta$) declines rapidly as the base-collector junction draws substantial current. To design a NOT gate like that, you need to know how many other gate inputs you want to hook up to your NOT gate output. 1? 10? 100? It matters. Try reading this about RTL design. – jonk Apr 4 at 7:13
• You need to know the output impedance of the source (plus any load impedance between it and the transistor's collector ; R2 in your linked schematic). – user_1818839 Apr 4 at 13:34
• is this along the lines of what you are asking? i.postimg.cc/ZKSBxWdG/Collector-current-in-saturation.jpg – Sredni Vashtar Apr 5 at 22:33
• @SredniVashtar yes – Schach21 Apr 7 at 18:59

The definition of saturation is that the base emitter and base collector junctions both are forward biased. This means that for an NPN BJT, Vbc will be positive. So the real answer is that as soon as you apply 1kV or something to the collector, the transistor will not be in saturation any more.

You can connect voltage sources of 1kV or 10kV but you have to limit the current flowing in the base of the BJT by a resistor R.

R is not an ordinary resistor because it has to work at 1kV or 10 kV.

Alternatively, you can use 10 ordinary resistors in series.

It's not strictly correct saying that a BJT has its own saturation current. There are actually many saturation currents.

Every BJT has a family of Vce and Ic curves, as a function of Ib, within which we define saturation regions.

Given a reference circuit with fixed Rb and Rc, Vce and Ic are functions of Ib.

Let's increase Ib. We assume that the BJT has saturated when Vce goes below a certain value, typically 50 mV.

If you increase Ib, the BJT will go into deep saturation and Vce will reach its limit. It will never go to zero Volt though.

I got my own rule to put small signal transistors into saturation: set Ib to 1 mA or more.

If Ib goes below 1 mA small signal transistors may work in the active region.

BC847, 2N4904 are widely used small signal transistors and if they all go into saturation with Ib = 1 mA.

BJT power transistors go into saturation with currents bigger than 1 mA. You should read the datasheet.

Example

VCC = +3 V

Ib = 1 mA

LKV:

3 - Rb*Ib - 0.7 = 0

Ib = 2.3 V / 1 mA = 2.3 k

Since I assume that the BJT has saturated, I know write the second LKV:

3 - RC*Ic - VceSat = 0

Assume VceSat = 0

I got now 2 degrees of freedom: Rc and Ic.

Open the datasheet. Find the VceSat curve as a function of Ic.

Figure 6, Page 8

https://assets.nexperia.com/documents/data-sheet/BC847_SER.pdf

Ic = 1 mA is fine in that figure.

Rc = 3 V / 1 mA = 3 k Ohm

• I don't think this really answers the question. – Hearth Apr 4 at 14:47

... they say the collector saturation current is entirely dependent on an external source.

Not only... If the transistor is saturated, its output collector-emitter part can be thought as a short connection - a closed switch or just a piece of wire with zero resistance and zero voltage across it.

Then the output circuit consists only of a voltage source (Vcc) supplying a resistor (Rc)... and it obeys the well-known (since the 19th century) Ohm's law.

So the (saturation) current Is depends both on Vcc and Rc:

Is = Vcc/Rc

doesn't the transistor has its own collector current saturation?

No, the (saturated) transistor does not have "its own collector current"... The current in the output circuit does not depend on the transistor if it stays saturated.

This is an extremely simple electrical concept since the 19th century that answers the general question, "Does the current in an electrical circuit depend on the closed switch with which we have switched on the power supply?"

Of course, the current will not depend on the closed switch... if it really is a switch with zero resistance and zero voltage across it. The current will depend only on the supply voltage and load resistance.

But if for some reason (for example, significant current, overheating, damage, etc.), the contact resistance and, accordingly, the voltage drop across the switch begins to increase, the current will begin to depend on it as well…

Similarly, the collector current Is of the saturated transistor does not depend on the transistor if the base current is sufficient (> Is/beta) to provide this collector current.

What happens if the source is 1kV or 10kV?

Nothing bad will happen at such a high voltage if Rc is high enough (for example, if Rc = 100 k, only 10 mA current will flow) ... and, of course, if the transistor stays saturated. Otherwise, it will be damaged by too much power dissipation (when it is semiconductive) or by too high voltage (when it is off). But we need both on and off states...

• Just for clarification (and to avoid misunderstandings): It is the VALUE of the collector current in saturation mode which depends on Vcc and Rc. Another question is under which conditions the BJT would be in this saturation mode. This depends. of course, on the base-collector pn junction which must be forward biased (like the base-emitter junction). As a good indication for this mode: A base current which is much larger than in "normal" quasi-linear amplification mode (at least 10% of the collector current). – LvW Apr 5 at 10:17
• A saturated transistor does not have "zero voltage across it". You need to be more careful with the language here. – Elliot Alderson Apr 5 at 11:28
• @Elliot Alderson, You forgot to mention that the supply conductors do not have zero resistance and zero voltage drop across them... – Circuit fantasist Apr 5 at 12:11
• The important consideration is that I did not say "the supply conductors and connectors will have no resistance". – Elliot Alderson Apr 5 at 12:44