For many, determining a BJT switch if correct is a hard task to do. It requires deep knowledge in electronic circuit theory and lot of effort. It seems only an advanced practitioner can do so. This is not really the case. Even a beginner may be able to learn to do so by following helpful tips from experts. This topic showcases an effective and short method how to know if a BJT switch is correct by looking at the circuit. The steps are fairly simple which includes identifying the biasing technique, checking the applied VBE and checking for saturation.
In this article, I will not anymore discuss the basics of BJT. If you are interested to learn it, read
NPN Transistor Principles and Practical Uses and How to Select a Transistor.
There is another type of a BJT and this is a PNP transistor. It can also be used as a switch, particularly a high side switch. But this article will not focus to this type of transistor. You can read
How to Saturate a PNP Transistor.
Step 1. Identify the Biasing Technique
There are few biasing techniques for BJT circuit. These are fixed bias, emitter bias, voltage divider bias and collector to base bias. Out of the four, there is only one biasing technique that perfectly fits for a switch application. It is the fixed bias.
The reason why fixed bias is the only one perfectly suitable as a switch is because it has no resistor in the emitter. With this, it is easy to drive the BJT into saturation and cut-off. As a switch, the BJT must be driven to saturation and cut off.
Both the emitter bias and voltage divider bias are out because it has emitter resistor. The presence of the emitter resistor makes the circuit difficult to drive to saturation. The collector-base bias is not an option also because the connection at the collector to the base resistor cannot be removed. In a switch configuration, the voltage source going to the base circuit is derived from a separate source, not from the same Vcc as the circuit diagrams above showed.
A fixed bias switch can be configured as any of the two options below:
Option 1 is the simplest form. Option 2 has the added resistor from the base to ground. The purpose of this is to force the BJT to turn-off in case the signal applied to the Vbb is floating when it is at off state. With this reason, option 2 is the most preferred switch circuit configuration.
Step 1 was just about memorizing these circuits, and they are easy to recall.
Step 2. Check the Applied VBE
Since the fixed bias circuit is the preferred configuration. So, the next step will only focus to options 1 and 2 circuits.
A BJT has VBE requirement or specification. VBE is the voltage drop across the base and emitter junction when the transistor is operating. For silicon material, it is around 0.7V. It is around 0.3V for a germanium material.
The applied VBE is the actual voltage provided to overcome the VBE requirement. For option 1, it is easy to check the applied VBE. Just take the Vbb level and subtract it with the VBE requirement of the BJT. The difference must be a positive value.
For option 2, remove the BJT in the circuit. Do a voltage divider across Rb2.
Voltage at Rb2 = Vbb x Rb2 / (Rb1+Rb2)
Take the result and subtract with VBE. The difference must be a positive value.
Shortcut for Step 2
- Option 1
If the Vbb level is higher than 1.5V, the VBE requirement is surely satisfied.
Why 1.5V? In some BJT, the maximum VBE specified in the datasheet is as high as 1.2V. To give an example for the VBE value in the datasheet, below is a part of the datasheet of PHPT61003NY-QX BJT from Nexperia. The VBEsat max is 1.2V.
- Option 2
A Vbb level of 1.5V is still applicable. However, the ratio of Rb2 to Rb1 must be 10 or higher. Such as Rb2 = 10k and Rb1 = 1k.
In actual application, BJT switch circuit is used to be driven by a signal coming from a microcontroller. Usual microcontroller output voltages are 1.8V, 3.3V and 5V. Thus, the BJT VBE requirement can surely be achieved.
Step 3. Check the BJT if Can Saturate
This is the challenging portion on how to know if a BJT switch is correct, but it is not impossible to do this, even without deep knowledge of electronic circuit theory.
If the applied VBE will able to turn on the BJT, the base current will set the operation of the BJT. The target operation is saturation because the circuit is intended to function as a switch. To operate at saturation, the base current must be big enough. But how big it must be? This is what we want to define here.
Below calculations looks too much for beginners. But don’t worry, there is a shortcut method. Keep reading below.
Long and Accurate Method
Base current calculation
- Option 1
Ib (base current for option 1) = (Vbb – VBE) / Rb
- Option 2
Ib (base current for option 2) = [((Vbb x Rb2) / (Rb1+Rb2)) – VBE] /[(Rb1xRb2)/(Rb1+Rb2)]
Collector current calculation
Next is to compute the collector current. The collector current for both option1 can be computed as:
Ic = Beta (minimum) x Ib
(where beta minimum is found in the BJT datasheet)
Since both base and collector current are derived, the status of the VCE can be determined. To say that the BJT is at saturation, the computed VCE must be less than the maximum saturation VCE defined in the BJT datasheet. For quick judgement, any computed VCE equals to 0V and lower (negative value) means the BJT is saturating.
The equation to use to determine the VCE is: VCE = Vcc – Ic x Rc
Shortcut Method
When a BJT saturates, ideally the voltage drop between collector to emitter is 0V. The level of VBE could also be ignored and these conditions simplifies the calculation. There is still a calculation but just a short one.
- Option 1
For a transistor to saturate, below equation must be satisfied
Vcc x Rb / (Vbb x Rc) <10
Why 10? It is widely adopted good practice that to achieve hard saturation, the circuit beta must be 10 or less. The circuit beta referred here is just the ratio of the collector current to the base current assuming the BJT is already saturated.
- Option 2
The method used in option 1 is still applicable but ensure the ratio of Rb2 to Rb1 must be 10 or higher. The equation will become
Vcc x Rb1 / (Vbb x Rc) <10
Actual Example on How to Know if a BJT Switch is Correct by Just Looking at the Circuit
Example circuit 1
Above circuit is an option 1 switch. The Vbb signal is provided by MCU. The MCU logic high level is 3.3V.
- Checking the Configuration
The circuit is correct in terms of configuration. It is a fixed bias derived.
- Checking the VBE
The BJT can turn on because the Vbb is 3.3V (higher than 1.5V).
- Saturation criteria
Vcc x Rb / (Vbb x Rc) <10
12V x 1k / (3.3V x 10k) = 0.36 (0.36 is less than 10)
Example circuit 2
Below circuit is a switch option 2.
- Checking the Saturation
The circuit is option 2, fixed bias derived switch.
- Checking the VBE
Since the ratio of Rb2 to Rb1 is 20 (this is more than 10), then a minimum Vbb of 1.5V is still applicable here. Thus, the BJT can turn-on without doubt.
- Saturation criteria
Vcc x Rb1 / (Vbb x Rc) <10
12V x 1k / (3.3 x 10k) =0.36 (this is less than 10)
Summarizing What We Learned
1. To make a BJT switch, use fixed bias configuration
2. Fixed bias could be configured with or without a pull-down base resistor
3. The BJT needs to be turned on
4. To turn on the BJT, the applied VBE voltage must be higher than the VBE requirement specified in the data sheet
5. For a quick determination if a BJT can turn on, a Vbb value of at least 1.5V is good indicating value for both options 1 and 2, but for option 2, the ratio of Rb2 to Rb1 must be at least 10
5. BJT switch must operate as saturation and cut-off
6. To drive the BJT into saturation, the base current must be big enough
7. To know if a BJT circuit indeed saturates, the VCE must be lower than the saturation VCE specified in the datasheet
8. For quick determination of saturation, below equations are applicable:
(Option 1) Vcc x Rb / (Vbb x Rc) < 10
(Option 2) Vcc x Rb1 / (Vbb x Rc)<10, considering the ratio of Rb2 to Rb1 is at least 10