Signal lines in analog circuit PCBs serve several important functions. They carry input signals, feedback signals, output signals, and reference signals. Given the various roles these signal lines play, each type of signal line must be optimized according to its specific function and the particular demands of the application. Despite this, the general rule remains: shorter signal lines are better. The longer the signal trace, the higher the chances of adding unwanted inductance and capacitance, which can interfere with the signal quality.
However, achieving the shortest possible signal path for every line is impractical. This is why the first priority when designing the PCB layout should be to identify and focus on signal lines that are most likely to be affected by interference.
In particular, the wiring of signal lines in the following circuits requires special care:
1) **High-frequency amplifiers and oscillators**: These circuits are sensitive to signal integrity, as high-frequency signals are more prone to attenuation and distortion from long or poorly routed signal lines.
2) **Multi-stage amplifiers**: Particularly those with higher output power, these circuits can generate significant noise and require careful routing to prevent interference between stages.
3) **High-gain DC amplifiers**: In these circuits, even minor distortions or noise can be amplified, so keeping signal lines as short as possible is critical to maintaining signal purity.
4) **Small signal amplifiers**: These circuits are designed to amplify weak signals, which means that any additional inductance or capacitance from long signal paths can introduce significant noise or distortion.
5) **Differential amplifiers**: For differential signals, it is essential to maintain balanced and short paths to avoid introducing common-mode noise or degrading the differential signal quality.
In conclusion, while it’s impossible to make every signal line the shortest, the key to successful PCB layout for analog circuits is prioritizing the most sensitive signal lines—especially those likely to experience the most interference. Careful planning of these signal routes, combined with optimizing trace lengths and minimizing their impact, will ensure the integrity and performance of the analog circuit.
### 1. **High-Frequency Amplifier PCB Layout**
When designing the PCB for a high-frequency amplifier, improper wiring can severely limit the amplifier’s bandwidth. The primary issue arises from the formation of large parasitic capacitors between closely spaced ground and signal traces. These capacitors, in combination with the output resistance, create a low-pass filter that restricts bandwidth. Additionally, if the input and output signal lines run in proximity, feedback can induce oscillations, further destabilizing the amplifier. To mitigate these issues, it is critical to maintain adequate spacing between these lines, ensuring that capacitive coupling is minimized.
PCB designers often face challenges with high-frequency amplifiers that tend to oscillate unexpectedly. A similar phenomenon occurs in oscillator circuit layouts, where the intended frequency of oscillation is not achieved. These issues typically result from capacitive coupling between signal lines. Therefore, reducing the capacitive coupling between traces is essential to avoid such problems and ensure stable operation of the amplifier or oscillator.
### 2. **Multi-Stage Amplifiers with High Power Output**
For multi-stage amplifiers, long power and ground traces can lead to low-frequency oscillations due to the resistivity of the wires. High currents generated by the power output flow through these traces, contributing to instability. A practical solution to this issue is to implement a power supply decoupling circuit by adding a sufficiently large capacitor between the power and ground planes. Alternatively, separating power and ground traces for each amplifier stage ensures that there is no shared path for the power and ground, preventing oscillations and improving stability.
### 3. **High-Gain DC Amplifiers**
High-gain DC amplifiers are typically used for amplifying small signals. When components like transistors or DC amplifiers are soldered onto the PCB, thermocouples may form at the junction between the device leads and the copper traces, generating alternating voltages and introducing interference signals. To minimize this effect, it is essential to stabilize the temperature around the input stage. This can be achieved by surrounding the input section with an isolation device that prevents airflow-induced temperature changes. Keeping the temperature constant around the input stage significantly reduces thermal noise and interference.
### 4. **Small Signal Amplifiers**
Small signal amplifiers are designed to process very weak signals. There are two main types of small signal amplifiers: high-impedance (low current) and low-impedance (low voltage) amplifiers, each with specific layout considerations.
#### (1) High-Impedance (Low Current) Amplifiers
In high-impedance circuits, capacitive coupling between adjacent signal lines can severely affect performance, potentially masking low-level signals. To minimize this coupling, it is essential to maintain a significant distance—typically at least 40 times the signal trace width—between high-impedance signal lines and other sources of interference. Furthermore, the ground capacitance of low-level signal lines should be kept high, which can be achieved by placing the signal lines close to the ground plane. If maintaining this distance is not possible, a ground trace can be placed between the signal lines to further reduce coupling.
When using photodetectors or chemical batteries as power sources, the power source impedance can be extremely high—up to hundreds of millions of ohms. If the PCB is inadequately cleaned after etching, leftover electrolyte can generate high resistance between adjacent traces. Even after cleaning, leakage resistance may still exist. To minimize the impact of leakage, it is advisable to surround the input of low-level converters with a guard loop, connected to a potential point that matches the general ground. This setup helps to neutralize the effect of leakage resistance and minimizes its impact on the circuit.
In high-impedance designs, avoid using plated through holes (PTHs), as the resistivity of PCB materials is generally lower than that of the surface, and PTHs can introduce unwanted leakage paths. Instead, it is recommended to connect high-impedance amplifier terminals to PTFE (Teflon) insulators rather than directly to PCB traces.
#### (2) Low-Impedance (Low Voltage) Amplifiers
In low-impedance circuits, inductive coupling or external magnetic fields can induce voltages that cause interference. To reduce this, the following measures should be taken:
1. Maintain a sufficient distance between high-level AC signal lines and low-level signal lines.
2. Route a ground trace near the signal traces to shield them from external interference.
3. Avoid forming ground loops, which can act as antennas, picking up external noise and affecting low-level signals.
### 5. **Differential Amplifiers**
Differential amplifiers are designed to amplify the voltage difference between two input signals while rejecting common-mode signals. If the PCB layout and differential amplifier design are not carefully executed, common-mode interference can create a differential signal, especially at low signal levels. The high input impedance of the differential amplifier means that any imbalance between the input traces can introduce significant noise.
To ensure proper differential amplification, the PCB design must be symmetrical, with the two input signal traces routed in a balanced and identical manner. This helps to minimize parasitic imbalances and noise pickup. Additionally, leakage resistance at the input can introduce an offset voltage, further affecting signal integrity. To address this, a protective device can be added to the input circuit, surrounding the signal line and ensuring that both ends of the differential input are at the same potential. This reduces voltage offsets and improves the accuracy of the differential signal processing.
The input circuit’s protective device should form a complete loop around the signal traces from the input to the amplifier connection point, maintaining a consistent potential across the circuit. For small-signal differential amplifiers, using epoxy-glass substrates can also help reduce leakage currents, providing additional stability to the circuit.
### Conclusion
In PCB design for various amplifier circuits, careful attention must be paid to trace layout, signal coupling, and thermal stability. Whether dealing with high-frequency amplifiers, multi-stage power amplifiers, or small-signal differential amplifiers, the goal is to minimize parasitic effects like capacitive coupling and leakage resistance. Proper spacing, shielding, and component placement are key to ensuring optimal performance and preventing common issues such as oscillations, signal interference, and thermal instability.
