When designing and manufacturing a product, a printed circuit board (PCB) can quickly become one of the most expensive parts, therefore any effort to bring down the cost of the PCB could have a significant impact on the final cost of manufacturing the product. Cutting down on PCB size, selecting the right parts, and employing the right design approaches will drastically reduce PCB expenses. In light of this, this article will discuss some of the most common strategies for reducing PCB costs during the design and manufacture processes. Various PCB design strategies, including, have been discussed in our previous posts.
What is Virtual Ground and Virtual Short?
Before we get into the intricacies, let’s take a look at the figures.Figure (a) depicts a short circuit that has caused VA’s voltage to equal VB. In (b), a short circuit has developed because the lines connecting VA and VB have been severed. There must be a virtual connection between the two sources, or some other virtual effect, for VB to equal VA even when there is no physical connection between them. The term “virtual short circuit” is commonly used to describe this occurrence.
Similarly, in Figure (c), even though the VA is wired to a 5V source, there may be effects such that the VA and VB both read 0V (Gnd_Potential).
The foregoing specifics might appear fantastical or implausible to you. However, the fundamental Op-Amp operations adhere to the aforementioned two ideas, and realizing why this is the case will aid in grasping the whole physics of Op-Amps.
Basic Op-Amp working Rules:
The Basic Op-Amp working mechanism mainly follows below given 2 important rules
- Both the Op-Amp’s non-inverting and inverting inputs must always have the same voltage. For consistent operation, the Op-Amp’s internal design and the output feedback resistors tend to balance each other out.
- According to its specifications, an operational amplifier (Op-Amp) has a lower output impedance and a larger input impedance. Therefore, optimal Op-Amp operation is achieved by assuming that there is no current flowing through the Op-Amp’s input terminals.
Virtual Short in Op-Amp
With a 1V input and a gain resistance of R1 = R2 = 1K, the circuit below is an example of the well-known non-inverting Op-Amp topology. In order to determine the connection between the input and output voltages, it uses a set of predefined equations. The output voltage can be calculated without resorting to the previously mentioned stated formulas by instead using the Basic Op-Amp principles.
For this circuit, V_Non_Inverting = 1V satisfies Rule -1, which states that the Inverting Input (-) voltage must be equal to the Non-Inverting (+) input voltage. The Inverting (-) terminal voltage is set to 1V by the Op-Amp VOUT, just as the Non-Inverting (+) terminal, which is not coupled to any dedicated voltage source.
When an operational amplifier (Op-Amp) is “Powered On,” its internal parameters are adjusted so that the Inverting Input voltage is 1V and, by definition, no current flows via the Inverting pin. With VOUT serving as the source voltage, R1 and R2 transform into a voltage divider whose output must be equal to the non-inverting input voltage.
In order to equalize the voltages V(+) and V(-), the output voltage is adjusted higher or lower.(-). Since R1 = R2 in this situation, we have a voltage divider with (V+) = (V-) at VOUT = 2V.
Following is the gain equation for a non-inverting operational amplifier:
Therefore, in Non-Inverting Operational Amplifiers, the Inverting Pin Voltage is equal to the Non-Inverting Voltage via a phenomenon known as “Virtual Short in Op-Amp.” This phenomenon does not require a physical short circuit between the two terminals.
Virtual Ground Concept in Op Amp
Following rules 1 and 2, the Inverting Pin of the op amp below should have no voltage applied to it. However, R1 links the inverting (-) pin to 5V. According to Rule 2, all current must flow through Resistors 1 and 2, with none passing through the Inverting (-) Input. The V(-) must be compensated for by voltage from the VOUT, making it equal to zero.
For the given circuit to work, the VOUT must be -5V (because R2 is also 1K) so that the voltage at the inverting terminal is 0. Changing the R2 Value will cause the Op-Amp’s internal structure to adjust VOUT so that V(In-) = 0.
Op-Amp in inverting configuration: VIN/R1 = -VOUT/R2
In this Inverting Input Configuration, the Inverting input is never actually connected to ground (due to the Non-Inverting Input Ground Potential), but it is always referred to as such. Despite receiving power from a 5V source at the inverter’s input, the inverter’s terminal voltage is still “Gnd,” hence the name “Virtual Ground” or “Virtual Earth.”
Importance of Virtual Ground and Virtual Short in Op-Amp
When analyzing an Op-Amp circuit, the Virtual Ground and Virtual Short are the two most crucial factors to consider. These two ideas form the basis of most transfer function formulations and Op-Amp circuit derivations, simplifying the circuit analysis by ignoring the Op-Amp’s input parameters.
Only “Closed Loop” Op-Amp circuits can use the Virtual Ground and Virtual Short Concept. There is no feedback mechanism to regulate the matching between the inverting and non-inverting input voltages in Open Loop or the Op-Amps used as comparators. Since VGVC is ineffective, the op-amp must remain permanently in saturation state. To prevent problems of the Op-Amp, the designer must consider the “Differential Input” Voltage restrictions under certain conditions.
When the output matching limit goes outside the Op-Amp’s Vcc and Vee supply ranges, the Virtual Ground and Virtual Short notions become null and void.