The Importance of the Q Point of a Diode to Circuit Functionality

December 5, 2019 Cadence PCB Solutions

Diodes arranged in a semi-circle

 

We all strive for that optimum point in which our performance, drive, and passion for what task lies before us converge. For example, working for the perfect company, that also appreciates your work, pays you accordingly, and the passion for what you do is unwavering.

The Q point we all seek is often elusive, and for some, it is unobtainable. However, it is human nature that promotes us all to continuously strive for the perceivably unachievable. Besides, if not for this unquenchable drive, we humans would not have accomplished the advancements in science you see today.

While on the subject of advancements, the majority of our accomplishments are within the field of electronics and engineering. It is only logical, especially considering the fact that these advancements provide the most noticeable, widespread, and life-altering effects on our lives. Furthermore, in the field of electronics, the landscape is continuously changing, and very few things that are true today that were also true even a few years ago. However, the operating point of a diode (Q point), happens to be one of those exceptions.

What is a Q point of a Diode?

Q point or the operating point of a device, also known as a bias point, or quiescent point is the steady-state DC voltage or current at a specified terminal of an active device such as a diode or transistor with no input signal applied.

Also, the Q Point is the relationship between the diode forward voltage and current defined by the device characteristic. Consequently, there is only one point on the dc load line where the diode voltage and current are compatible with the circuit conditions.

In other words, this operating point (Q point) is the intersection where the optimum forward voltage and forward current converge, and it is also the point where the diode operates at its optimum.

The Importance of a Q point to Circuit Operation

Another way of expressing a Q point is by addressing it as biasing. Biasing in electronics means establishing predetermined voltages or currents at various points of an electronic circuit to promote proper operating conditions in electronic components.

Numerous electronic devices, including diodes, transistors, and even vacuum tubes, whose function is processing time-varying AC )signals, also require a steady DC current or voltage to operate correctly. Furthermore, when supplying the AC signal, you do so by superimposing it on the DC bias current or voltage.

Whereas, a bias circuit is a portion of the device's circuit which supplies this steady current or voltage. In summary, the Q point is essential to a component’s (diode) or circuit’s overall functionality.

Calculating the Q Point of a Diode

As previously discussed, the operating or quiescent point (Q point) is the point at which the diode operates optimally. Furthermore, the Q point of a diode is specific to the circuit in which the diode is apart, therefore it is necessary to calculate it for the particular circuit you are designing. In general, it is the circuit designer who picks the Q point to operate the diode at a safe nominal current.

To calculate the Q point of a diode, we must use Kirchhoff's voltage law (KVL), which states that the algebraic sum of the potential differences in any loop must be equal to zero (ΣV = 0). For example, if two resistors say R1 and R2 are wired together in a series connection, they are both parts of the same loop, so the same current must flow through each resistor.

Now, let’s consider that a diode is also in series with a resistor (R1), and a source voltage (VDD). We will connect the diode in forward bias so that the forward current and the forward voltages will flow through the series circuit. As per the Kirchhoff’s current law, the current flowing through the diode (ID) and the resistor (IR) are equal (ID = IR).

Printed circuit board with diodes along with vias and other components

Understanding the Q point of a diode enables better circuit current functionality.

 

Further Analysis of the DC Load-Line and the Diode Q Point

We can now further analyze the circuit by applying Kirchhoff’s voltage law (KVL). KVL results in the formation of the final equation for the dc load line. Here the dc voltage is the biasing voltage of the circuit by keeping any further reactive components as zero. After applying KVL to the series circuit, we can obtain the voltages and currents of our circuit.

(VDD = VD +IDR)  (VD = IDR - VDD)

In regards to the formula, VDD is the applied dc source voltage, and VD is the voltage across the diode. Therefore, you can consider the above formulation as the equation for the diode. We can now obtain the voltage and current characteristics of the diode in its forward bias condition. Thereby, as with our previous analysis on the condition of the diode in forward bias, the applied voltage and the generated current in the circuit are exponentially related to each other.

Also, upon reaching a specific cut-off voltage, the diode starts operating in its forward bias condition. Now that we have a characteristic curve (graphically), we can now finish our analysis technique and draw a straight line on the i-v characteristics curve. The curve for our generalized circuit for the diode is (VDD/R). Thereby the intersection of the lines results in the formation of the diode operating point (Q point) as previously mentioned.

The Overall Significance of the Q Point in a DC Load Line

As you may know, diodes are non-linear components that do not follow the conventions of Ohm’s Law. However, if you analyze its i-v characteristics curve, it is still based on voltage and current parameters. During this graphical analysis, we plot the characteristic curve, and then we draw a DC load line. This, of course, represents the point where reactive components are zero.

This also represents the constraint applied to the non-linear component by the external circuit. From here, we, of course, define our Q point. So, what is the significance of setting this Q point and using this analysis technique? The answer is as follows:

  1. It allows us to obtain a linear analysis for non-linear components like diodes and transistors.

  2. The primary purpose of the analysis of the load line is to ascertain the operating point.

  3. The operating point created by the dc load line is the center where the voltage and current parameters are equivalent to each other for both the parts of the circuit.

  4. The operating regions created here because of the load line analysis method ensure the diode remains in an active region.

  5. The operating point we obtain is also essential to drawing the ac load-lines.

  6. If the constant resistance and applied dc voltage to the circuit vary, then this technique is vital for efficiently analyzing the circuit.

PSpice simulation of output voltage of Zener diode

Finding the expected output voltage with a Zener diode can be made easy.

 

The Q point is essential to the overall component and circuit functionality. It ensures that non-linear components like diodes operate at their optimal current and voltage throughout the operating range. This also promotes increased functionality, reliability, and life cycle of your electronic circuits.

Implement proper Q point operational strategies for all of your PCB designs with Cadence’s suite of design and analysis tools. OrCAD PSpice Simulator can not only obtain Q point calculations for all of your electronic circuit design needs, but also provide simulations and models for expected voltage outputs and power supplies. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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