BC549C Transistor

The BC549C transistor is a NPN style transistors. The transistor is very common and made by a lot of manufacturers. all transistors has a cross reference check with the most common characteristics:

Property	Value
Polarity	NPN
Max Collector Power Dissipation (Pc)	0.5W
Max Collector-Base Voltage (Vcb)	30V
Max Collector-Emiter Voltage (Vce)	30V
Max Collector Current (Ic max)	0.1A
Max Operating Junction Temperature (Tj)	150°C
Transition Frequency (Tj)	200MHz
Collector Capacitance (Cc)	4.5pF
Forward Current Transfer ratio (\(h_{FE}\))	Min 420

The “C” in “BC549C” refers to the \(h_{FE}\) category of the transistor. The \(h_{FE}\) value can change dramatically between transistors, even within batches of the same manufacturer (this is why match pairs is in some cases such an important thing!). “A” is the lowest \(h_{FE}\) category. Without a reference to a category, it might be anything within the range for that particular transistor type.

As BJT transistors are current controlled (and not voltage controlled like tubes and FET transistors), we use a current source to get the characteristics curves, using the following SPICE model:

Note that these curves are only valid for a particular \(h_{FE}\), so they might be much different per transistor: that’s maybe the reason that such curves are not part of the regular datasheets?

To check the actual \(h_{FE}\) of an NPN transistor, we can use the following circuit:

The value for \(R_1\) and \(R_2\) are chosen to make it easier to translate the voltage at \(V_{probe}\) to the \(h_{FE}\) gain. Assuming a voltage drop over \(V_{be} = 0.7V\), the actual voltage drop over \(R_2\) will be: \(9.0V - 0.7V = 8.3V\), so the base current will be: \[\dfrac{8.3V}{2.2MΩ} = 3.773uA\] We won’t measure the current at the collector, but we can measure the voltage drop over \(R_1\). So with \(R_1 = 2651Ω\) this corresponds nicely, as \(2651 * 0.3773 = 1000\).