Tech Notes

Applies to Models 7224, 7548 and 7796

Compensating the Amplifier to the Load for CC Operation

This article describes the method for determining and setting proper compensation when operating an AE Techron 7000 Series amplifier in Controlled Current mode. For basic information about Controlled Current and Controlled Voltage modes of operation, please see the Tech Note "Controlled Voltage vs. Controlled Current Modes of Operation."

Controlling Compensation for CC Operation

AE Techron 7000 Series amplifiers can be configured for either Controlled Voltage (CV) or Controlled Current (CC) mode of operation. When operating the amplifier in Controlled Voltage (CV) mode, compensation is not required. However, when operating in Controlled Current (CC) mode, the amplifier load becomes an integral part of the system. In order to ensure system stability and to control available bandwidth, compensation via an RC network is required for CC operation. The following steps will allow you to safely and effectively compensate your amplifier for operation in CC mode.

STEP 1: Check Amplifier Operation in CV mode.

We recommend that you power-up and enable the amplifier in Controlled Voltage mode without attaching a load before configuring your amplifier for Controlled Current operation. This will allow you to verify that the input signal and the amplifier are operating correctly.

Once this initial check is completed, power down the amplifier and access the amplifier main board to place the amplifier in CC mode. (Consult your product manual for CV/CC jumper location.)

One of two compensation settings can be selected via jumpers on the main board: CC1 which enables the factory-installed RC network (see Figure 1), or CC2 which allows installation of a custom RC network.

STEP 2: Determine Required Compensation

When operating an amplifier in Controlled Current mode, the load becomes an integral part of the system. In order to determine the required compensation for your load, begin by consulting the following table to determine the approximate compensation capacitance (C) required based on the inductance of your load:

 

Compensation Capacitance
(Cc)

Load Inductance (L)
<200 microHenries > 200 microHenries
to < 1 milliHenry
>1 milliHenry
0.001 microfarad 0.01 microfarad 0.1 microfarad

NOTE: Load Resistance (R) is assumed to be <5 ohms.

STEP 3: Determine if Default or Custom Compensation is Required

7000-Series default RC network for compensation

Figure 1: Factory-installed
default RC network.

If your load inductance is between 200 microHenries and 1 milliHenry and your load resistance is less than 5 ohms, then you can likely use the default compensation provided by the amplifier's factory-installed RC network. To select the factory-default compensation, please see STEP 4 below.

If your load inductance falls outside of the mid-range, or if your load resistance is greater than 5 ohms, then you must calculate your required compensation. If, after calculating your required compensation, you determine that the default compensation will be insufficient for your load, then you will need to enable and install a custom RC network. See STEP 6 below.

STEP 4: Enabling Your Compensation Setting

AE Techron 7000 Series amplifiers can be enabled with one of two compensation settings: default RC network or custom RC network. The required network can be selected via jumpers on the main board. CC1 enables the default (factory-installed) RC network, while CC2 allows installation of a custom RC network. Figure 1 describes the default RC circuit.

To select CC1, place jumper J5 in the UP position; to select CC2, place jumper J5 in the DOWN position. (See Figure 2 for jumper J5 location.)

IMPORTANT: If CC2 is selected, you must calculate the compensation requirements for your custom RC network and install the network on your amplifier main board before operating the amplifier in CC mode.

J5 and J6 jumper locations

Figure 2: J5 Jumper Location

 

STEP 5: (Optional) Verify Suitability of Default Compensation (CC1)

If desired, the following values of the components contained in the default RC network can be used with the formulas provided in STEP 6 below to verify the suitability of the default compensation for your uses.

Pins Jumped Compensation Resistor Compensation Capacitor Parallel Capacitor

1 & 2

UP

R63

68 K-ohms

C16

0.047 microfarad

C11

47 picofarad

 

STEP 6: Installing an RC Network for Custom Compensation

If the default RC network does not provide suitable compensation for your intended load, you will need to install a custom RC network that is matched to your load. This network will require two components (a resistor (R) and a capacitor (C)) to be installed on the main board. To calculate the approximate values required for each component, use the fomulas provided below.

COMPENSATION FORMULAS:

To find the value for the resistor (Rc) in the RC network:

Rc = 20,000 x 3.14 x L x BW

where:

Rc is compensation resistance in ohms.
L is load inductance in henries.
BW is bandwidth in hertz.

To find the value for the capacitor (Cc) in the RC network:

Cc = L/ (R x Rc)

where:

Cc is compensation capacitance in farads.
L is load inductance in henries.
R is resistance of load in ohms.
Rc is compensation resistance in ohms.

 

STEP 7: Optimizing the Compensation Values

Once an approximate Rc and Cc have been computed, these values will need to be evaluated. To do this, install components with the required values in the main board at locations R82 and C25 as shown in Figure 3.

Custom compensation component locations

Figure 3: Custom compensation component locations.

 

Remember the load you are connecting is a part of the system and the amplifier should not be turned on without the load being connected.

After installing the components, check to ensure that jumper J5 is correctly installed (see STEP 4), then power up the AE Techron amplifier without signal input.

To begin testing, input a square wave with a frequency of 100 Hz to 1 kHz, or a squared pulse at a low level (typically 0.25 to 2.0 volts). A limited-rise-time, repetitive pulse of low duty cycle is preferred.

Observe the output current through a current monitor or current probe. Look for clean transition edges. The presence of ringing or rounding on the transition edges indicates compensation problems. (See Figure 4.)

 

Compensation effects on waveform

Figure 4: Compensation effects on waveform.

 

Squarewave showing need to decrease R

Figure 5: Squarewave
showing a decrease in
R is required.

Squarewave showing need to increase R

Figure 6: Squarewave
showing an increase in
R is required.

Squarewave showing need to increase C

Figure 7: Squarewave
showing an increase in
C is required.

If a change in compensation is necessary, an adjustment to the resistor component of the Compensation circuit is probably required.

If the output current waveform is ringing, the circuit is underdamped: You have too much gain and should lower the resistance (see Figure 5).

If the output current waveform is rounded, the circuit is overdamped: You have too little gain and should increase resistance (see Figure 6).

If the output current waveform is neither underdamped or overdamped, but the top of the squarewave is not level, then you should instead increase the capacitor value (see Figure 7).

When making adjustments:
Resistor: Increase or decrease resistance values in increments of +/- 10%.
Capacitor: Incrementally increase capacitor values by a factor of 2 or 3.

After final adjustments have been made to the circuit, the final waveform for your planned application should be tested to confirm the amplifier's compensation setting.

NOTE:

  • If possible, use 1% metal film resistors. AE Techron discourages installation of potentiometers in the resistor location of the compensation circuit because this can decrease stability and may increase inductance.
  • The parallel capacitor in the RC network serves to increase stability, but can be removed, if it is not required for system stability. If the parallel capacitor is used, it will usually decrease the value of resistance needed.
  • In multiple amplifier systems, expect to decrease the value of R63 in series systems by 1/2.

For additional assistance in modifying amplifier compensation, please contact AE Techron Technical Support at tech@aetechron.com.