● Learn about the use of cascaded amplifiers.
● Gain a greater understanding of when a cascaded amplifier is needed.
● Learn more about calculating cascaded amplifier gains.
Single amplifier forming a section of the cascaded amplifier circuit.
During the height of car audio, many considered the increasing size of subwoofers as the next breakthrough in sound output (SPL). But, like nearly all things in the field of electronics, advancements are staggered due to current ancillary limitations.
The increase in driver size created the need for an increase in amplifier power. However, the amplifier technology at the time did not match the pace of the advancement and subsequent increase in subwoofer size. This introduced the car audio world to daisy-chaining (cascading) to accommodate the need for increased amplifier output.
What Is a Cascade Amplifier?
Daisy-chaining (cascading) amplifiers in the field of car audio is no longer necessary due to the advancement in amplifier technology, i.e., increased SNR, efficiency, stability down to ½ ohm, and output power. In other areas within the field of electronics, cascading is still a requirement.
As you may know, a cascade amplifier is a two-port network comprised of a series of amplifiers in which each amplifier connects (sends) its output to the input of the next amplifier in the chain. This complicates gain calculations for these cascaded stages due to the loading between the stages or.
The performance requirement of many applications is unobtainable from a single-stage amplifier, thus the need for multiple-stage amplification. These cascaded amplifiers produce increased gains over the gains possible by the individual amplifiers. In general, the overall gain of a cascade amplifier is the result of the gains of the individual stages, ignoring the potential loading effects.
The Purpose for Cascading Amplifiers
The overall reason for cascading amplifiers is the need for an increase in amplifier output to meet a specific requirement, e.g., to increase the signal strength in a Television or radio receiver.
Using a cascade, or multistage, amplifier can provide your design with a higher current gain or voltage gain. Typically, we utilize cascading amplifier stages to increase our overall amplifier gain, but in other instances, it is for achieving a necessary input or output impedance.
Below is a simplified view of a cascade amplifier with two stages in series.
Here we have a simplified diagram of the same two-stage cascaded amplifier in circuit-level view.
Cascaded Amplifier Gain
We briefly referenced that calculating the overall gain of a cascaded amplifier is more complicated due to the loading between the amplifier stages. Keep in mind that these are still amplifiers, and therefore, individual output gains will fall under the purview of amplifier gain characteristics.
Amplifier gain correlates to the relationship between the measure of the input signal to the ratio of its output signal. There are three types of amplifier gain in which we can measure: current gain (Ai = Iout/Iin), power gain (Ap = Av * Ai), and voltage gain (Av = Vout/Vin). This depends on the quantity we measure, but in any case, A (amplification) is the representation of gain.
Av = Voutput/Vinput
Av = (40 volts RMS)/(10 volts RMS)
Av = 4
With this in mind, the gain of a cascade amplifier is the product of the gains of its individual amplifier stages, aside from possible loading effects. However, the gain of each stage or amplifier individually relies on its configuration, i.e., its components. The formula for a cascaded amplifier gain is as follows:
A (Gain) = A1 * A2 * A3
When the gain of each stage uses the decibel expression (dB), the sum of the gains of the individual amplifiers is its total gain:
A (Gain in dBs) = A1 + A2 + A3
Benefits and Advantages of Cascaded Amplifiers
When we cascade an amplifier, there is a requirement to utilize a coupling network amongst the amplifiers. We call this type of coupling interstage coupling. With cascaded amplifiers, there are three cascaded amplifier types: direct coupling, transformer coupling, and RC coupling.
RC coupling: affords the lowest cost for implementation and provides an acceptable frequency response.
Transformer coupling: affords enhanced total gain and level matching impedance. Functionally, it expands its signal across the primary transformer winding and performs as a load. However, this method can be costly if utilizing a wide frequency response transformer.
Direct coupling: the coupling of the output of one stage of the amplifier to the input of the next stage. This permits signals with zero frequency (direct current) to pass from input to output. This is ideal for applications requiring zero or low-frequency amplification.
There are two primary advantages of cascade amplifiers: increased gain and input, and output impedance flexibility. The need for the gains provided by cascade amplifiers is paramount to the functionality of various applications.
Whether you are designing a custom multistage amplifier for a specialized signal chain or you need to simulate cascaded amplifier gain and efficiency, you will need the right set of PCB layout and design software. Allegro PCB Designer, and Cadence's full suite of design tools, can help you create your cascaded amplifier from verified component models and then analyze all aspects of its functionality. You'll also have access to a set of tools for MCAD design and preparing for manufacturing.
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